Now that I think about it, there is an idiom or two in Leafoosish that is in standard usage:
ffêr yn arianrod - literally, "to do an Arianrhod". It means "to abandon/push away/mistreat", usually in reference to children/kittens, and it comes from Arianrhod's treatment of her son Lleu Llaw Gyffes. (This is not considered very nice to do.)
foler ("to fly") is used idiomatically in the same sense as English "to work" or French "marcher", namely "to function correctly". So if you say es l'fheityr folant, it doesn't mean "*the car is flying" but rather "the car is working".

There are a couple of idioms in Meikar. But since the language is still in heavy development, it's not really a focus.

"To give somebody a piece of their death," or "Akh+<person> megan gan Shaonapan<*> bo yezo" is basically fighting words for whoever you imply it to. In English would be pretty much the same as saying "beat the **** outta someone!"

So, "Akhden megan gan shaonapanden bo ayezotei!" is literally "To his possession a piece of his death I will give." or "I will give him a piece of his own death!"

So dem's fahtin' wurds! >:/

*Possession can be implied in two ways, "X gan Y," or "X+pronoun-suffix."

There's a few from Síyähärä:

Ös'rä þäk - literally 'of that time', and usually used to translate 'once upon a time', it's often used to indicate sarcasm or disbelief in the likelihood of an event.

Dän dürnduď šlänø ví hístä/hísta - literally 'the day dances by him/her', meaning the person is not paying attention or is drunk.

Dän Nrätšø zërnön ärgön - literally 'the Gods are [in] abundance' meaning it's not been a nice day.

In Tikolmian, dlicos zo lesit (lit. to show/uncover the sun) means "to state the obvious".

For example: Dlicos zo lesit mic i isos sōz si lamotl! [uncover-INF the sun-from to-1PL REL on-2SG need NEG sit/set-3SG.PRS] (There does not sit a need on you to show us the sun = you don't need to state the obvious)

I have an idiom(I think) cyadzvnoz pīdzechbvuyan īltrovu -the dark god bites back-karma

I haven't yet worked out most of the idioms in Experimental Conlang A, but I have identified some euphemisms in preparing a Swadesh list and determined that it has both simple compounds and kennings.

On the Swadesh list, "penis" and "testicle" have a formal register used in a medical sense and euphemisms used in everyday

Penis:
Medical: ɹuf-g (m)
Euphemism: dæŋ-p (muscle, m)

Testicle:
Medical: vlaiHa-to (m)
Euphemism: kibik-st (sack, f)

"Excrement" has polite and impolite registers, but no euphemistic meaning.

A sample kenning:

oʔi fju-p oʔifju-p
fly flower butterfly (lit."flying flower")