User:Kirklansdon
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The explanation one of my acting coaches gave was that the expression was born in the time of William Shakespeare. When thespians completed a performance, during curtain call they would first take a polite bow that would not require that they bend a knee or “break a leg”. If the audience insisted on a second curtain call, they would return to the stage and take a deep bow that would require them to “break a leg”. So break a leg was a wish that the actor have an amazing performance. I don’t have a reference to offer. I don’t even remember my instructor’s name, but the definition works for me.