Break a leg is highly disputed, though it's narrowed down to one of a couple probable things.
Both relate to the renaissance. In that time it basically did very well. For one of two reasons.
Either, break a leg meant curtsy, and when you saw the queen you got to curtsy for her. So do well enough to see the queen.
It may have also done to do with the way a curtsy was carried out. It put strain on the back of your leg. The better you did the longer the audience clapped. The joke was to do so well that the audience would clap so long your leg would break while doing a curtsy.
It had nothing to do with karma. That much is sure. That definitely a misinterpretation.
