Said to actors (or performers in general) for good luck before they go on stage, especially on an opening night.
Theatrical types are well known for their belief in superstitions, or at least for their willingness to make a show of pretending to believe in them. The term ‘break a leg’ appears to come from the belief that one ought not to utter the words ‘good luck’ to an actor. By wishing someone bad luck, it is supposed that the opposite will occur.
Many possible origins:
To “break the leg” or “break a leg” is archaic slang for bowing or curtsying; placing one foot behind the other and bending at the knee “breaks” the line of the leg. In theatre, pleased audiences may applaud for an extended time allowing the cast to take multiple curtain calls, bowing to the audience.
In the time of Ancient Greece, people didn’t clap. Instead, they stomped for their appreciation and if they stomped long enough, they would break a leg. Or, some would have it that the term originated during Elizabethan times when, instead of applause the audience would bang their chairs on the ground—and if they liked it enough, the leg of the chair would break.
In the time of Ancient Rome, gladiators would fight to the death as a form of popular entertainment in the colosseum. Spectators would sometimes shout “quasso cruris,” the Latin equivalent of “break a leg.” This essentially would be wishing them “good luck” by requesting they keep their lives and only cripple the other opponent by breaking his leg.