These are idioms I use in my custom setting, but some may find them useful. Some of the idioms are more inspired by the world's lore. Without further ado:

'Common' Idioms

• Hungry enough to eat the north end of a south-bound stallion
• Stinks enough to gag a goblin
• Haven’t got the sense to tell a bishop from a bugbear
• Hot as a goblin ass in a pepper patch
• Can’t see the bottom of an empty mug - drunk
• Like pissin’ in a wicker basket - Impossible/ useless
• Kenku see-Kenku do - one who imitates without understanding
• By the Sun/Void! - Interjection of extremely happy/disgusted surprise, respectively (the names of two Pantheons)
• Grab the dwarf by the beard - Start a fight for no good reason
• Tin-cuppa - A bar too rowdy to use glassware
• Crusties - undead (especially afflicted by the Charophage, a curse that plagues the land)
• Rusties - Warforged
• Not here to drink dragon wine - here to drink cheap, low quality liquor
• Could wilt the ears off an elf - very strong liquor
• Growing scales on his ass - acting uncharacteristically strong or brave
• Got the charm of a succubus/incubus - charming or overtly sexy female/male
• Can’t carry a tune in a minecart
• Crown - slang for a gold coin
• Too many crowns and not enough headroom - more money than sense
• Wear his crown on his seat shiner - an idiotic authority figure
• Melt some metal - splurge or spend extravagantly
• Dusting coffers - Very poor, in debt
• Good news for the doctor is bad news for the Undertaker - (or vice versa), everything is a matter of perspective

Dragonborn idioms

• A Long winter makes a sweet wine - endure hardship for later rewards
• Done X more than you've got scales - said by someone with a lot of experience
• Look before you sneeze - Be cautious and courteous of others nearby
• Tasting your own smoke - being haughty and self-indulgent
• A single bad grape won't spoil the wine - look out for your neighbors, look on the bright side
• Have scales of many colors - be very open-minded and experienced (my dragonborn worship Ghanjat, who is a plane-travelling dragon god with each scale a different color, reflecting a plane he's visited)
• Don't let your breath burn your tongue - Don't act cocky
• Tasting food in a story - fantasizing about something you lack
• You can't breathe two elements - Don't set expectations too high, focus on one task
• A Dragon won't remember being an egg - you'll grow out of something
• One wing fighting the other - When two parties cannot cooperate and make life harder for themselves
• Dragons inviting a sheep to dinner - taking advantage of someone
• Like Egg, Like Scale - It’s hard to change oneself
• Have claws harder than one’s scales - one who can’t take criticism
• Frost breath won't uncook a chicken - what's done is done

Dwarf Idioms - Dwarven idioms tend to be either battle, mining, or mead related

• Fire in my beard! - Interjection of surprise or horror
• By Hamrongeid's hammer! - Interjection of surprise (Hamrongeid is Dwarven for the Smith god)
• Swinging true - something that ‘sounds right’
• No problem a hammer can’t fix!
• Clay-headed / mud-headed - pejorative for ignorant, naive, or stupid
• Sharp as a fresh pick - clever or efficient
• Like twice drank ale - something that tastes terrible; the implication is that one drank ale, then pissed, then drank it again
• Like folding stone - something impossible
• Like sorting gold from gophers - something trivia l or easy
• Like a two-ton Auroch - big and/or strong
• So long as Aevilog burns bright - forever (Aevilog is an eternal flame that powers all dwarven forges)
• Even a wet stone is dry on the inside - encouragement to withstand hardship
• You can mine the biggest mountains - nothing is impossible
• A bit of heat coaxes the steel - one can encourage or motivate using threats
• Even a dull/goblin axe can kill - 1) Make do with what you have, 2) Don’t get caught unprepared
• Mixing honey in my mead - Acting obsequiously, or trying to use excessive flattery
• Serving mud in a mead hall - 1) making an egregious or stupid mistake, 2) being dishonest
• No hammer’s too big to hit X - 1) any foe/problem can be overcome, 2) No such thing as “overkill”
• Landslides don’t ask directions - 1)It’s better to be direct, 2) There’s no planning around unstoppable forces the
• Pissing steam/ Mad enough to piss steam - very angry
  *Going up the tall side of the mountain - doing things the hard way

Elven Idioms - Usually inspired by nature, hunting, and with more historical references dud to their age

• Hunter’s Blood! -Vulgar Interjection of shock, surprise, indignation (Hunter is a deity sacred to most Elves)
• On wilted ears - Hard of hearing, naive
• Giving the dragon’s share - being stingy, telling half-truths
• Poor Rooting - Pejorative for extremely low-born, either half elves or human (in Elven - cirtlag)
• When Illydri returns (to the light)! - Quite vulgar or taboo interjection, meaning never (Illydri were a despotic elven dynasty, banished and became drow)
• Sacred Soul! - An interjection of praise and pleasant surprise (reference to Sublime Weave the coalescence of elven ancestral memory)
• Ni Linn - Neutral term for non-elven races (typically other humanoids) (lit. ‘not us’)
• Weaving Lilies - 1) Selling an elaborate yet obvious fiction, 2) being obsequious
• Earn the envy of the songbirds - beautiful
• Visiting Giants - daydreaming, concocting fantasies
• Judging a tree by its bark - making specious assumptions
• An eagle by day, An owl by night - 1) Use the right tools for the right occasion, 2) act according to your strengths
• A sleeping forest may have many eyes - watch for unseen threats or eavesdroppers
• Arrows in the wind - subject to the whims of fate
• May you outlast your sword! - a caution for the overconfident
• By blade or by bow - finding a solution no matter the problem
• Fullest he that spares the hare - mindfulness and compassion is better than self-indulgence
• A whisper in the wind - Something going against great odds
• Tongues like fangs - using language that’s curt, insulting, or hostile
• Hoofs but no horses - something notable in its absence
• More than one belly for an arrow - don’t underestimate a foe (the side of a bow with the string is a ‘belly’)
• Firing Dry - An attempt ending in embarrassing failure (no arrow released when firing a bow)
• Stringing a bow after the hunt - 1) Too late, 2) plan ahead
• Fletch-marked - having a reputation for being bellicose or aggressive (implication that they’ve fired too many arrows and their bow is worn)

Goliath Idioms

• Stories ripe for retelling - history repeats itself
• Have a Giants' reach - overly ambitious possibly at great risk
• A mountain has two views (from the top and from the bottom) - 1) those who overcome a problem may have a new perspective on an issue, 2) meant to encourage people to overcome adversity
• Ask the Giants - You're asking questions you don't want the answers to, or that nobody knows
• Being friends with you echo - Enjoying solitude over company with others
• A mountain grows from the bottom, and shrinks from the top - a nod to the passage of time, and transfer of power between generations

Gnome Idioms

• Quit Braiding my ears! - quit teasing me
• Shiny - something amazing that you like/ want
• Building a burrow from the inside
• Conjured Mead won't drink the same
• Hair on all sides- a generally unpleasant situation
• Get your eyes stuck on X - Become obsessed or stare at something
• Gold for the tinsmith - something too fancy, you wouldn't know what to do with it
• Thinking in knots - confused
• Silence is a good friend (in dangerous times) - being stealthy is a preferred stratagem
• Longer than a gnome wedding - a very lengthy affair
• Go sell it to the Lyres - Tell your unbelieveable story to a bard and not me (Lyres for Hire is a guild of bards and other entertainers who handle bookings in taverns and publish a small weekly newspaper)

Halfling Idioms - Often rustic, agricultural, or inspired by their nomadic tribal lives

• As the hawk flies - In a straight direction, ignoring terrain
• Having turnips in your ears - deaf or dumb
• Knee high to a ram - small or young
• Fewer years than feathers - having confidence in spite of inexperience, due to youth
• Locking horns with a ram - stubbornly sticking to an idea or path
• Picking flowers in the briar patch - 1) seeing good in a bad situation, 2) wasting time
• Pitching a tent in the rain - doing something too late or in inappropriate conditions
• Living with one’s lips between the ‘skin and the pipe - meaning living a life of hedonistic luxury (‘skin is also a double entendre for wine or promiscuity)
• Not a foot too big to rest by our fire! - hospitality to other races (common follow up - “or too bald!” said teasingly)
• Footpatch - hair on top of the feet
• Getting briars/fleas in one’s footpatches - Getting anxious, restless or having wanderlust
• Down to the fescue - running low on resources (fescue is a grass that makes grazing animals sick)
• Goblin in a goatskin - being dishonest about your intentions
• One sausage short of a farrow - not having enough of something, usually because if wastefulness or impulsiveness (farrow is a litter of piglets)
• Making/sleeping in a grass bed - getting kicked out of the family tent for shameful behavior
• Trading skins - moving to a new tribe because their totem matches your values better
• Having skin that doesn’t fit - not fitting in with the rest of the tribe because you lack in the qualities associated with the tribe’s spirit animal (can be very insulting)

Orc Idioms - Orc idioms tend to be violent and somewhat laconic

• Cowardice is no excuse to go on living - 1) don’t be a coward, 2) it’s better to fail than to never try
• An elf/dwarf in the crosshairs - An obvious solution
• Draw blades, draw blood - Only draw a weapon you intend to use
• Even a dragon knows to look down - be as humble as is practical
• Fighting a dragon from the mouth down - taking a position of strategic disadvantage
• There’s more than enough blood to spill - 1) not caring about spilling a bit of blood as a means to an end, 2) even though one may risk personal harm they feel confident in their success
• Death is too late for X - it’s time for that person to die
• Eating with the Nalashaddin - 1) engaging in unsavory or violent work (Nalashaddin are very violent warriors that others are too afraid to eat with or sleep beside)
• Fangs aren’t for speaking poems - there’s no point in mincing words
• Axes aren’t for planting - 1) Use the right tools for the job, 2) what you can’t grow, steal
• At least we can still bleed - An optimistic look at surviving a failure
• Take off your own armor - Survive long enough to doff your armor, don’t let yourself become loot
• Living on black broth - Subsisting on the bare minimum (black broth is a poor-tasting but nutritious food for travelling armies)
• Invaders fear only the poor - There’s no negotiating with somebody who has less than you
• Wanting for dust in the desert - 1) never satisfied, 2) living in extreme hardship
• Borders grow a sword at a time - progress occurs in small strides
• It’s not a grave that contains my memory - glorious deeds live on in stories, a life without these is not worth remembering anyway
• A thousand swords makes one army - An army should fight as cohesively as possible
• As easy as being surrounded - there are many possible solutions

Tabaxi Idioms

• Beware the beautiful flowers (they are often poisonous) - Avoid those who abuse flattery
• The jungle is listening - Be careful what you say (a borrowing from Elven)
• Draw a bow with two hands - Being foolish and impractical
• When the lion judges, all goats are guilty - (also used in the reverse) to speak ill of someone who is biased or set in their ways
• The snake doesn't choose his camouflage - basically, beggars can;t be choosers
• An early scar is better than a later one - get failure out of the way (also a reference to a tradition of ritual "scarring" of kits born on a solstice)
• Don't eat now the quarry of tonight - plan only with resources available to you in the moment
• The tea of others is not too hot for you? - don't worry about what others do or have so much
• short-maned - Shamed or recently defeated, also inexperienced (manes are shaved if one loses in ritual combat)

1 My setting's nations aren't so much designed to be primarily populated by one race because I wanted to break the "fantasy zoo" trope where the world is broken up into an elftown, humanland, dwarfburg, etc. So, these idioms are more about the cultures of my world, but they may be more appropriately adapted as racial sayings in other settings.

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From comments:

I am a sucker for fantasy idioms that aren't just english idioms with words swapped out! These are great!

Some of the ones I use are here, but they're not tied to any particular race:

• “You ought to/Hold your sword by the hilt” - Keep it simple, stupid.
• “You’ll never know the length of a vein until you mine it.” - You never know until you try
• “At night, be the shadows.” - When in Rome
• “A still forest means trouble” - a ‘sailor’s warning’ but about land wilderness.
• “A bad situation is like a bad ale. Best to drink it fast so you don't taste it long, then to prepare for a long night of regret. And maybe next time you'll check the label.”
• “You could sharpen an axe on his head” - when someone is really stubborn.
• “If you don’t have a dog, hunt with a cat” - Make the best of your situation
• “Leave your temple in the village” - don’t exaggerate
• “Don’t praise the day before the evening” - Don’t take things for granted too early on
• “Even squirrels can fall from trees.” - Sometimes even experts are wrong.
• “Don’t jump ditches the long way” - don’t do things the hard way.
• “Don’t start the house with the roof” - You need to do things in the right order.
• “They have more wool than a lamb” - Someone is RICH.
• “Never request more than one song from a bard” - Don’t push your luck
• "Flies can't enter a closed mouth" - Stop talking.
• "The splinter is part of the branch" - similar to "like father, like son"