Since you asked:
Stage actors are a particularly
superstitious lot -- they don't say "Macbeth" in a theater, they don't whistle backstage, and they never wish each other "good luck" before a performance. This last tradition is a form of reverse psychology. They don't want to tempt fate by talking about positive outcomes in advance. So instead, they tell each other to "break a leg." While the idea behind the phrase is quite old, possibly dating from medieval belief in
malevolent spirits, "break a leg" itself is fairly
recent. It was whispered in theater circles starting in the 1920s, and first appeared in print around 1954 or 1957.
The exact origin of "break a leg" isn't clear. Several etymology resources note the phrase's similarity 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.
Now you know why breaking a leg is a good thing...on stage, that is.