The origin of the term “break a leg” in the acting world is much disputed. What is known is that it is a wish of good luck, applied only to actors, and not to other theater workers. Stagehands are basically told, “Don’t mess up,” though the word “mess” is often replaced with a more popular four-letter word.
The superstitious interpret the phrase as a way to discourage evil spirits from deliberately causing one’s performance to suffer. Actually wishing someone good luck on the other hand, would be evoking the evil eye. Thus expressing “good luck,” may actually cause bad luck for the actor.
The term may be traced in origin to Elizabethan language. To break a leg in Shakespeare’s time literally meant to bow, while bending the knee. Since only a successful actor would bow on stage and receive applause, this phrase would have, in effect, been a wish of good luck and good performance to the actor. However, in the 16th century, the phrase also meant to give birth to an illegitimate child, which is hard to connect to the theatrical world.
Others trace “break a leg” to the tradition of the audiences of Classical Greece. Instead of applauding actors, audiences would stomp their feet. Stamping to the point where one would actually hurt himself is unlikely. Vigorous stomping sounds, however, expressed greater appreciation for the performance of the actor. Causing people to pound their feet so hard that they would injure themselves would indeed signify accomplishment in acting.
More rooted in the tradition of superstitious reasons for wishing that someone might “break a leg,” relates to John Wilkes Booth leaping to the stage at the Ford Theater after firing the shots that would assassinate President Abraham Lincoln. In the jump, he actually broke a leg. However, his performance as an assassin is clearly not enviable, unless it is later connected to the comedians’ concept of “killing” an audience, which means being successful.
The saying may also be attributed to the film and play 42nd Street. In the script, Peggy Sawyer is given the opportunity to play the lead in the production because she is the understudy of the star who actually breaks her leg and cannot go on. Peggy’s performance is hailed as a success.
Landing parts is often termed as getting a “break” in show business. Being successful is referred to as breaking into show business. The term “break a leg” may also then be connected to one making a successful entrance into the world of acting.
Another possible construction is the German phrase Hals un Beinbruch, translated to "happy landings" in English. Both German and English pilots used the term, which translates literally to breaking all one’s bones. Actors may have adopted this, as the phrase was clearly in use in the 1920s after WWI.
Ballet dancers have their own version of of the phrase, which connects to the superstitious concept of not wishing other dancers good luck. They say merde! which literally translates to a well known four letter word for excrement in English. This term seems more expressive of not evoking ill-luck but as well may imply feelings related to stage fright or anxiety about performance.