It is said that stage actors are a very superstitious. They don’t say “Macbeth” in a theater, they don’t whistle backstage, and they never wish each other “good luck” before a performance. They tell each other to break a leg because they don’t want to tempt fate by talking about positive outcomes in advance.
Break a Leg was heard in theater circles starting in the 1920s, and first appeared in print around 1954 or 1957. The idea behind the phrase is quite old, possibly dating from medieval belief in malevolent spirits, but break a leg itself is fairly recent.
The exact origin of “break a leg” isn’t clear. The phrase is similar to a German saying “hals und beinbruch”, meaning “neck and leg break.” It’s used to mean good luck. One theory is that German-speaking or Yiddish-speaking Jews brought the saying with them to America early in the 20th century. Many of these immigrants worked in theater, so the translated phrase spread.
Some say the expression comes from before the movie era in the 18th and 19th century where a performer doing an exceptional job in a scene in a play would occasionally stop the show for a moment as the audience broke into appreciative applause for his or her performance. The actor or actress would then modestly come forward to the footlights and acknowledge the ovation with a bow or curtsy, in other words, bending the knee, or “breaking the leg.” Once the audience’s demand was met, the performer would get back in character, rejoin the other actors in the scene and the play would continue.