Some idioms sound alarming but mean the opposite. 'Break a leg!' — a wish of good luck — is thought to come from where?

"Break a leg" is widely believed to originate in theater culture, where wishing a performer "good luck" directly was considered a jinx, so actors developed the superstitious habit of wishing the opposite instead. Saying something ominous was thought to trick fate into delivering the good outcome, a common pattern in superstitions across many professions.

The phrase has nothing to do with an actual leg injury or a running race — those interpretations take the words too literally and ignore the theatrical superstition that actually explains the idiom's backwards logic.

Theater is full of similar reverse-superstitions, such as never wishing an actor "good luck" directly or avoiding saying the name of Shakespeare's "Macbeth" inside a theater, both reflecting the same instinct to avoid tempting fate by stating a wish outright.