A crook is someone who makes a living by being dishonest or breaking the law. When a person steals, tricks others out of their money, or commits crimes regularly, people might call them a crook. The word suggests someone who habitually does wrong, not just someone who made one mistake.

In old detective stories and gangster movies, you'll hear characters say things like “That crook robbed the bank!” or “The police finally caught the crook.” Bank robbers, con artists who run fake schemes, and thieves can all be called crooks.

The word has an old-fashioned, almost playful sound to it today, though the crimes themselves are serious. You're more likely to hear it in classic movies or books than in modern news reports, where people use terms like “criminal” or “thief” instead.

Crook also has a completely different meaning: a bend or curve in something. A shepherd's staff has a crook at the top, curved like a candy cane, useful for guiding sheep. When you bend your arm, you create a crook in your elbow. “The crook of your arm” is that bent space on the inside of your elbow where you might carry books or hold a baby.