In theater and other performing arts, we say “break a leg” to wish performers good luck as they go on stage. This phrase originated in the Elizabethan era  when theater was the prime art form. At the time, many performers were superstitious. They believed that wishing someone good luck would “jinx” them, and it would result in bad luck. At first, people would avoid expressing any wish altogether. In the same era, the audiences would often bang their chairs on the floor instead of applauding. The more they liked the performance, the more they banged their chairs, and that would often result in someone breaking the leg off a chair or two. 

Soon, someone figured out a way to express a wish that sounded negative but really had a positive meaning that only theater insiders would know. Performers started telling each other,  “I hope you break a leg,” which sounded like it meant “I hope you get injured while performing” to avoid jinxing the performer. Both parties knew that it meant “I hope your performance is so well received that an audience member breaks a leg off a chair.”