1. Onomatopoeia: The most likely origin is that "boo" is simply an onomatopoeic word. This means it mimics a sound, in this case, the sound of a sudden fright or surprise. It's a natural exclamation that people make instinctively when startled or wanting to scare someone.
2. "Bough" or "Bough-woo": There's a theory that "boo" evolved from the hunting cry "bough-woo," used to scare or chase away animals. This is less likely, but interesting nonetheless.
3. "Boo-hoo": The word "boo" could also be a shortened form of "boo-hoo," which is a sound of crying. This might explain its use in playful teasing or mocking.
4. "Boob": Some speculate that "boo" may be related to the word "boob," meaning breast, and could have been used in a derogatory way to insult someone. This is less common and less likely.
In summary: While the exact origin of "boo" might be uncertain, its most likely source is its natural ability to imitate a sound of fright or surprise, making it a truly universal word.