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)

A friendly encouragement to performers prior to a show. The expression may derive from the idea of a performer, having left the stage, being called called back from behind the legs for an encore.