Where did the phase "Break a leg" come from?

Evidently, in the days of early vaudeville, the producers would book more performers than could possibly perform in the given time of the show - since "bad" acts could be pulled before their completion... so, in order to insure that the show didn't start paying people who don't actually perform, there was a general policy that a performer did NOT get paid unless they actually performed on-stage. So the phrase "break a leg" referred to breaking the visual plane of the legs that lined the side of the stage. I.E. "Hope you break a leg and get on-stage so that you get paid." - Josh Pritchard (jfpritch@mit.edu)

Or Maybe...

In the old days, actors were always told to "Break a Legend", meaning to do even better than performers before them. It ended up being shortened to "Break a leg". - Leslie Sidley