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"Boo" is a phrase used as an interjection that was meant to either surprise or frighten. “BOO” comes from the Middle English “bo”, same meaning, which was used in writing as early as the 1500s. Boo has been associated with Halloween and the noise a ghost makes when scaring someone. People attempting to scare others may hide and jump out exclaiming, "Boo."
A Brief And Spooky History of the Word ‘Boo’. (2016) Culture & Arts. Retrieved from https://www.huffpost.com/entry/history-of-the-word-boo_n_581761ede4b0390...