If you're running a campaign in a world or culture which is not our own, it becomes tricky to keep your idioms logical. So many idioms rely on the background of shared culture we have that it's easy to drop them into conversations where they actually make no logical sense being used.

• "It's a piece of cake" - what if the setting has no such thing as cake?
• "What on earth are you doing?" - what if the setting isn't on Earth?
• "He's on a sticky wicket" - what if the setting doesn't have the sport of cricket?

What to do? Purge them from allowable conversations taking place in the game? Well, you could try, but that way lies madness. It's better to retain natural language and just gloss over the problem.

Transcript

Luke: It looks like we Rebels pose a greater threat than you thought, Emperor.
Palpatine: The moon is dark, and the gods dance in the night; there is terror in the sky, for upon the moon hath sunk an eclipse foretold in no books of men or of earth's gods...
Admiral Piett: {over radio} Piett here, Your Most Reverential Highness. Interesting fact: Our star destroyers are now primed and ready to fire. Those Rebels don't stand a chance.
Luke: What?
Palpatine: Do not call up any that you can not put down.
Admiral Piett: We're not going to attack? Combat simulations show an 87% chance of unqualified success, rising to a 92% probability of sustaining only minor materiel losses.
Darth Vader: We have an even better option.
Palpatine: Madness rides the star-wind... claws and teeth sharpened on centuries of corpses... dripping death astride a bacchanale of bats from nigh-black ruins of buried temples of Belial...
Darth Vader: You might not want to get in the way, my son.
Admiral Piett: I hope this is better than your last so-called plan, Lord Vader.