"BREAK A LEG"
Not literally, Uncle Woody.
It is considered bad luck to wish someone good luck in a theater. In English-speaking countries, the expression "break a leg" replaces the phrase "good luck", which is considered unlucky.
The exact origin of this expression is unknown. One theory is that "Break A Leg" is a very old military term for "taking a knee", or bending down to one knee and breaking the line of the leg. In the theatre it is a reference to "taking a bow". To wish someone to "Break A Leg" is to ask them to give the best performance they are capable so that they may deserve to take a bow at performance end -- or, to "Break A Leg".
This expression has so entered the mainstream that it is used by non-actors toward actors and in non-theatrical situations, however, to say "break a leg" in ballet is considered just as much bad luck as saying "good luck", considering it's a physical art.
