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.”
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.”
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.
Informant – “You know the story of Macbeth. There are a lot of witches in that play. Legend has it that the curses that they say are real. If you say the name of the Scottish Play in a theater needlessly, that theater is cursed. The name summons the witches and curses. To reverse it, you have to run around three times in a circle and spit, or say your favorite curse word. You also get shunned by your cast, which is not fun.”
Informant – “I heard it from my freshman theater teacher. He was crazy. I said Macbeth in class once and he yelled at me ‘YOU NEVER SAY THE SCOTTISH PLAY’S NAME.’ He almost threw a chair at me.”
I can’t think of any practical application for this superstition, so I believe it exists to create a more complex theater subculture. If you know about it then you are more of an theater person than those who don’t.