I think it's safe to say that idioms are what trip up both language learners and conlangers the most, once they've got the grammar and some decent vocab down.
Post any idioms you have or know of here; natlang or conlang, all are equally interesting and useful. I'll try to remember to add some by tonight.

Salvaging from the old ZBB Idioms &c. thread, which it seems was mostly aphorisms and adages:
to kick the bucket
heart of gold
shoot the breeze

Maja:
Sarkart hoj endos.
You're dragon food.

It pretty much means, "You're fucked." However, 'sarkart', "dragon food" can also be used as an insult.

Some I came up with before for my conlang:
chew air – be poor
catch a fish with mere hands/teach a wall to part – try the impossible (from a sceptic viewpoint)
have eyes like sponges – be a crybaby
break someone's eyes – to delude someone
spit books – be a smartass
have bathed in knives – bleed everywhere; bleed to death
serve last week's leftovers (for the 3rd time) – bring up a topic that's been done to death

In short, no it's not.

It's pretty much exactly that, yeah.

Airumali:
mnagalan usseie "to play naga". To be lazy or unmotivated; to lay about.

And suddenly...

I learned yesterday that if someone's cheese is slipping off the cracker, it means they're going insane.

That seems pretty analogous to "becoming unglued" or "unhinged" perhaps? Allusions to the separation of things that were normally not separated.

Many Spanish speakers use pan de Dios ('loaf of bread of God') to refer to a really good person.

Tu esposo es un pan de Dios. - 'Your husband is really good person' (lit. "a loaf of God-bread").

We say "ser un trozo/cacho/pedazo de pan" [to be a piece/chunk of bread]

Bread is good I guess.

Communion. It's about communion. Or something.

mas bueno que el pan > more good than bread, a really good person
saco de huevas > sack of eggs, a dumb person.
mata de huevas > same, but mata means like... a cluster of plant matter
que te metan el pico en el ojo > for someone to put their dick in your eye, means that they've outsmarted you deceitfully, they've deceived you
volado > made to fly, flyed, given flight, means to be high on fun drugs
cagado > one who has shat themselves, means to be avaricious and averse to spending money, overly attached to money
manito the guagua > one whose hands are like those of a baby, means the same as cagado
pintar el mono > to paint the monkey, means to behave in a very attention-manner fashion, to be an attention whore
florerito > little flower vase: the same, but friendlier
pichicateao | pichi gato > piss of a cat, or he who has consumed piss of a cat, means for someone or something's performance to be enhanced by drugs. also cocaine
merca > from mercadería, means cocaine
pito > whistle, means a joint of marihuana
pucho > leftover in mapudungun, comes to mean cigarrette or the butt thereof in clsp
sacar la cresta > to take the crest off something: means to give someone a severe beating
partir la raja > to split open someone's buttcrack, means the same thing
queso de cabeza > head cheese, means smegma
pancito inflado > you know when bread's just out of the oven and yeast has made it puffy? well, we use that expression to describe a woman with a meaty mons veneris.

Stupid question: what do you call a phrase that doesn't have a non-literal meaning, but is still very common as a phrase, for example wait and see?

Set phrase.

Teal'c was good at stranging those, and idioms.

Most of the line about radio waves is from Dear Esther:

I will become a torch for you, an aerial. I will fall from the sky like ancient radio waves of flawed concrete. Through underground springs and freezing subterranean rivers. Through the bacteria of my gut and heart. Through the bottomless boat and forgotten trawlers where nobody has died. Like the hermit and Lot’s wife, I will fossilise and open a hole in the rock to admit me through.

It's sorta one of my all-time favourite free verse snippets, partly because of the bizarre vocabulary and qualifications, so it's not very surprising it ended up in Vokram untampered.

Salthan:

(note: masciline 2nd person is used here. Replace with "sani" to mean 1pf, tasa for 2pm or tosi for 2pf.

olanys daladsachsana chisud - He (who is thirsty) is looking at the sky.  To look at the obviously wrong place for something (it hardly rains in the desert)
etes dasan chiolrel foldaurel - To give a little light (candle) to the sun. To be boastful.
sanleSekhasaseknai - To have 24. To be short of perfection.
negak girelos |akaushe - When Hell heats up. Never. (Hell is cold to the cold-blooded Salthans).
has danakete - To be held by a cactus. To have someone do something that seems kind, but is betrayal.
darads dasanlenashe - To speak through one's nose. To be dishonest.

I heard someone say "[...] either freeze this fish or cook it," which sounds like a decent idiom. In Maotic I guess that'd be kontak loskó suptak tán lubei.

In Ntaratu, rimpak warpansunu "to shorten one's tongue", is used as a phrase meaning "suck up to superiors, try to seem more high-status than one is", referring to the annoying habit of native elites to replace their trills with retroflex flaps in imitation of the distinctive accent of Jalve speakers. A similar idiom is kuihek paryehenu, "brighten one's hair".