"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".

Source: Wikipedia