Break a Leg
Meaning: To wish someone good luck, especially in a live theater production.
Origin: There are several theories about the origin of this really popular phrase. The most popular one is that theater casts and crews are famously superstitious, believing that if you wish someone good luck, it’s tempting fate.
Source: Quick meme
Another theory takes into account the old meaning of breaking a leg, which is to curtsy or bow. So, it would be a wish for an actor to give such a good performance that he would be forced to take many bows, and thus break his leg. Another more technical theory is that the “leg” refers to curtain legs in a theater, which one breaks in order to arrive on stage. The term break a leg didn’t appear until the 1920s, in the United States.