"Break a leg" is an idiom in theatre used to wish a performer "good luck" in an ironic way. Well-wishers typically say "Break a leg" to actors and musicians before they go on stage to perform. The origin of the phrase remains obscure.
The expression reflects a theatrical superstition in which wishing a person "good luck" is considered bad luck. The expression is sometimes used outside the theatre as superstitions and customs travel through other professions and then into common use. Among professional dancers, the traditional saying is not "break a leg," but "merde".
Urbane Irish nationalist Robert Wilson Lynd published an article, "A Defence of Superstition", in the 1 October 1921 edition of the New Statesman, a British liberal political and cultural magazine. Lynd regarded the theatre as the second-most superstitious institution in England, after horse racing. In horse racing, Lynd asserted, to wish a man luck is considered unlucky, so "You should say something insulting such as, 'May you break your leg!'" Lynd did not attribute the phrase in any way to theatre people, though he was familiar with many of them and frequently mingled with actors backstage.
The earliest known example in print is from Edna Ferber's 1939 A Peculiar Treasure in which she writes about the fascination of the theater, "...and all the understudies sitting in the back row politely wishing the various principals would break a leg". In Bernard Sobel's 1948 The Theatre Handbook and Digest of Plays, he writes about theatrical superstitions: "...before a performance actors never wish each other good luck, but say 'I hope you break a leg.'" There is anecdotal evidence from theatrical memoirs and personal letters as early as the 1920s.
There are several theories behind the origin of the phrase. Few are supported by contemporary writings. The theories listed below are some of the more popular explanations.
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.