Content:
Informant – “Break a leg is when you wish to wish an actor good luck in a theater. You can’t say good luck, you instead say break a leg.”

Context:
JK – “When did you first hear it?”

Informant – “I heard it many years ago when I was performing and around stage people. They just told me that’s what you do.”

JK – “Why do you think this practice exists?”

Informant – “I was told that the gods of the theater, if you told someone good luck, the mischievous gods would intercede and that person would screw up. But if you didn’t say that, if you told them the opposite, the gods wouldn’t have anything to respond to and they wouldn’t notice that person. It could also be that the applause would be so great that you would have to take a bow. And the traditional bow, in the Victorian bow, you would have one leg in front of the other and to bow you would break your leg. Well not literally, but it would be like bending your knee in a weird way.”

Analysis:
There is also a humor to the wish. Before it became a cliche, telling someone to break a leg may have been a way to get them to laugh and relax before going on stage.