Break a leg

"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, likely first used in this context in the United States in the 1930s or possibly 1920s, originally documented without specifically theatrical associations. When said at the onset of an audition, "break a leg" is used to wish success to the person being auditioned. 
The expression probably reflects a superstition (perhaps a theatrical superstition) in which directly wishing a person "good luck" would be considered bad luck, therefore an alternative way of wishing luck was developed. 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 the French word "merde".
Non-theatrical origins.
German and Yiddish term.
Most commonly favored as a credible theory by etymologists and other scholars, the term was possibly adapted from the similar German phrase , literally "neck and leg(bone) break", itself borrowed from , , because of its similar pronunciation. For example, the autobiography of Manfred von Richthofen records pilots of the German Air Force during the First World War as using the phrase to ironically wish each other luck and safety before a flight. The German-language term continues to mean "good luck" but is still not specific to the theatre.
Horse-racing term.
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! 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.
Theatrical origins.
The aforementioned theory regarding , a German saying via Yiddish origins, suggests that the term transferred from German aviation to German society at large and then, as early as the 1920s, into the American (or British and then American) theatre. The English translation of the term is probably explained by German- or Yiddish-speaking Jewish immigrants entering the American entertainment industry after the First World War. 
The earliest published example in writing specifically within a theatre context comes from American writer Edna Ferber's 1939 autobiography "A Peculiar Treasure", in which she writes about the fascination in the theatre of "all the understudies sitting in the back row politely wishing the various principals would break a leg". American playwright Bernard Sobel's 1948 "The Theatre Handbook and Digest of Plays" describes theatrical superstitions: "before a performance actors never wish each other good luck, but say 'I hope you break a leg. There is some anecdotal evidence from theatrical memoirs and personal letters as early as the 1920s. 
Plausible theories.
During Elizabethan times when, instead of applause, the audience would bang their chairs on the ground — and if they liked it enough, banging harder on the floor, the leg of the chair would break. The performers would wish each other, "I hope you break a leg." 
Alternative meanings.
There is an older, likely unrelated meaning of "break a leg" going back to the 17th and 18th centuries that refers to having "a bastard / natural child".
Alternative terms.
Professional dancers do not wish each other good luck by saying "break a leg"; instead they say "Merde!", the French word for "shit". In turn, theater people have picked up this usage and may wish each other "merde", alone or in combination with "break a leg". In Spanish, the phrase is "mucha mierda", or "lots of shit". In Portuguese, it's ""muita merda", with the same meaning. This term refers to the times when carriages would take the audience to the theatre. A quick look to the street in front of the venue would tell if the play was successful: a lot of horse dung would mean many carriages had stopped to leave spectators.
Opera singers use "Toi toi toi"", an idiom used to ward off a spell or hex, often accompanied by knocking on wood, and onomatopoeic, spitting (or imitating the sound of spitting). Saliva traditionally was supposed to have demon-banishing powers. From Rotwelsch "tof", from Yiddish "tov" ("good", derived from the Hebrew טוב and with phonetic similarities to the Old German word for "Devil"). One explanation sees "toi toi toi" as the onomatopoeic rendition of spitting three times. Spitting three times over someone's head or shoulder is a gesture to ward off evil spirits. A similar-sounding expression for verbal spitting occurs in modern Hebrew as "Tfu, tfu" (here, only twice), which some say that Hebrew-speakers borrowed from Russian.
An alternate operatic good luck charm, originating from Italy, is the phrase ""in bocca al lupo!" ("In the mouth of the wolf") with the response "Crepi il lupo!" ("May the wolf die")
(see "Standard Dictionary of Folklore, Myth & Legend").
In Australia, the term "chookas"" has been used also. According to one oral tradition, one of the company would check audience numbers. If there were not many in the seats, the performers would have bread to eat following the performance. If the theatre was full they could then have "chook" —Australian slang for chicken— for dinner. Therefore, if it was a full house, the performer would call out "Chook it is!", which became abbreviated to "Chookas!" It is now used by performers prior to a show regardless of the number of patrons; and may be a wish for a successful turnout.
In Russian, a similar tradition existed for hunters, with one being told "Ni pukha ni pera" ("Neither fur nor feather") before the hunt, with the reply being "K chiortu" ("Go to hell"). Today, this exchange is customary for students before an exam.
In popular culture.
The 2001 Broadway musical comedy "The Producers" features a song titled "It's Bad Luck To Say 'Good Luck' On Opening Night," in which the novice producer Leo Bloom is instructed that the proper way to wish someone good luck on Broadway is to say "Break a leg." Moments later, the show's star is seen to break his leg—preventing him from performing—and in a later scene he breaks his other leg. The number also appears in the 2005 film version of the musical.