A sound made to show disapproval or contempt, often used by an audience. The crowd responded with boos when the referee made a controversial call.
A term of endearment for a significant other or close friend. She called her boyfriend her boo when introducing him to her friends.
A sound made to startle or surprise someone, often used in playful contexts. He jumped out from behind the door with a loud 'boo' to scare his sister.