A sound made to express disapproval or to startle someone, or to call out someone's name affectionately.
From Old English 'bu' or 'buh,' an imitative word meant to startle or frighten. The disapproving meaning developed later, possibly influenced by the sound's natural connection to surprise and alarm.
The word 'boo' is one of the oldest onomatopoeia in English—it literally mimics the sound you make to scare someone, which is why it works perfectly both at haunted houses AND at sporting events when you want to show disapproval.
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