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 superstition in it (you avert a bad event just by wishing it...).