It originates from old superstitions that to wish an actor good luck would tempt fate and encourage the Gods to cause bad luck as opposed to good. So this in turn has led to people to use negative expressions to wish their luck instead of positive messages. For example in France they say Merde! Which in English means Crap! The German expression is Hals und Beinbruch which literally means "neck and leg fracture" and leads people to believe that the English expression came from the German. It also may have stemmed from wishing someone a 'big break' as in good luck leading to success, this show could be their 'lucky break'! so to speak, but overtime it led to misinterpretation and the term 'break a leg!'. Another theory is that the successful French actress Sarah Bernhardt had only one leg and so to be as good as her would be very lucky.
