Boo is what people shout to startle someone or express disapproval.

When you jump out from behind a door and yell “Boo!” at your sister, you're trying to give her a harmless scare. It's a short, sudden sound that catches people off guard. Young children love to play peek-a-boo, covering their faces and then popping out with a cheerful “Boo!”

The word takes on a completely different meaning at sporting events or performances. When a crowd boos a referee's bad call or an actor's poor performance, they're showing their disappointment or disagreement. Unlike cheering, which shows approval, booing signals that people think something is wrong or unfair. A room full of people booing creates a chorus of disapproval that's hard to ignore.

Boo can also be a noun: you might hear boos from the audience, or someone might let out a loud boo at a villain in a play.

Interestingly, people sometimes say they wouldn't “say boo to a goose” about someone who's extremely timid or shy, meaning that person is so quiet they wouldn't even make a sound to frighten away a bird.