When you wish an actor good luck before he goes into an audition or out on stage to perform, you tell him to Break a Leg. Sounds like an odd way to wish someone luck, doesn’t it?
As actors themselves will tell you, we are a suspicious lot. For instance, you never tell anyone about audition you are going on or how you did at it because that will jinx it and you won’t get the job. Actors are not unique in that, many people believe in jinxes.
Well, telling an actor to do well or wish them luck is a jinx. It guarantees failure. So, you tell them not to do well so that they will, in fact, do well. So, you tell them to break a leg.
It is also said that break in this case means bend. As in curtsying or bowing low, as an actor would do if she receives thunderous applause.