Overview
"Break a leg" is a typical English idiom used in the context of theatre or other performing arts to wish a performer "good luck". An ironic or non-literal saying of uncertain origin (a dead metaphor), "break a leg" is commonly said to actors and musicians before they go on stage to perform or before an audition. Though the term likely originates in German, the English expression is first attributed in the 1930s or possibly 1920s, originally documented without specifically theatrical associations. Among professional dancers, the traditional saying is not "break a leg", but the French word "merde".
Non-theatrical origins
Yiddish-German pun theory
Most commonly favored as a credible theory by etymologists and other scholars, the term was possibly a loan translation from the German phrase , literally "neck and leg(bone) break", itself a loan translation from, and pun on, a Yiddish phrase, a wish for good luck, because of the Yiddish phrase's humorously similar pronunciation to the unrelated German phrase. For example, Luftwaffe pilots are reported as using the phrase to wish each other luck. The German-language term continues to mean "good luck" but is still not specific to the theatre.
Superstition theory
The 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, regarding the theatre as the second-most superstitious institution in England, after horse racing. In horse racing, Lynd asserted that to wish a man luck is considered unlucky and so "You should say something insulting such as, 'May you break your leg! Thus, the expression could reflect a now-forgotten superstition (perhaps a theatrical superstition, though Lynd's 1921 mention is non-theatrical) in which directly wishing a person "good luck" would be considered bad luck, therefore an alternative way of wishing luck was employed. Lynd did not attribute the phrase in any way to theatre people, but he was familiar with many of them and frequently mingled with actors backstage.
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