in bocca al lupo

literally it means “in the wolf’s mouth” in italian language, but it is an expression used to say “good luck” to someone who is about to face any kind of test/challenges/problems. the common response is “croak the wolf” (crepi il lupo!).

it was originally used by hunters as a way to ward off evil by wishing the exact opposite of luck (indeed “to be in the wolf’s mouth” means “to be at the mercy of the wolf”) and still today there is a bit of superst-tion in it (you avert a bad event just by wishing it…).
example 1:
a: “i have the math test tomorrow”
b: “oh really? good luck then…”
a: “you don’t say so, for it’s bad luck. it’s “in bocca al lupo”!”
b: “ok, sorry… in bocca al lupo!”
a: “crepi il lupo!”

example 2 (from “gotham” s01e01, don falcone is putting jim gordon on a hard decision: do what he says or go against him):
jim gordon: “you want me to keep quiet about all this?”
don falcone: “gotham is on a knife edge. what do you suppose bringing down city hall and the police force will do, even if you could? would it make things better? in bocca al lupo”