给出一个英文短语或俚语，然后要求解释其含义和用法。

break a leg
"Break a leg" is a common English idiom used to wish someone good luck, particularly in the performing arts, such as theatre or music. It may seem counterintuitive, as breaking a leg is a negative thing, but it is believed to have originated as a way to ward off bad luck by offering the opposite wish. The phrase can be used between performers or by well-wishers to show support or encouragement.