A superstitious and widely accepted alternative to "Good Luck" (which is considered bad luck). A more full derivation of the term can be found here.

a superstitious good luck wish exchanged by actors who feel that saying "good luck" is a jinx.

A superstitious and widely-accepted alternative to "Good Luck" (considered to be bad luck)

Said to wish someone "Good Luck" in a performance.

"Break a leg" is a well-known saying in theatre which means "good luck". It is typically said to actors before they go out onto stage to perform.