system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we yell "Boo!" when we scare people, and not some other word?
gpt
Okay, according to the OED, the first time it's mentioned anywhere is from a document in 1718. ("1718   G. Crokatt Sc. Presbyterian Eloquence iii. 111   Boo is a word used in the North of Scotland to frighten crying Children.") When something is that old, who knows WHY they might have said it, it just gets passed down, and passed down, and passed down, til, voila, we have no idea why. 

EDIT: found more on a Slate article: "Variations of the word boo—including bo and boh—have been found in books as published as far back as 500 years ago. While the Oxford English Dictionary notes the similarity between bo and the Latin boāre and the Greek βοãv, both meaning “to cry aloud, roar, shout,” it’s unlikely that bo and boo—as nonsensical exclamations—derived  from these words. An etymological dictionary of Scottish from 1808 notes that the sound  might denote "a sound in imitation of the cry of a calf," or be related to menacing creatures like the bu-kow and the bu-man (a possible ancestor of the modern bogeyman)."

Interesting question!