The Cut cites the Oxford English Dictionary's definition of "boo," which compares it to Latin and Greek words that meant "to cry aloud." But other writers have shunned that connection. Slate compares it, instead, to imitating the sound of a cow or a calf.
The use of the word "boo" has shifted some perceptions of how ghosts interact with people, according to Mental Floss. Before the word gained popularity, ghosts were usually seen as intelligent and pretty verbose. That interpretation is found in a lot of literary works, like Shakespeare's "Hamlet" and other pieces from earlier eras. In later works of art, ghosts seemed to lose some of those traits, opting instead for sounds like "boo" to scare the audience.
Taking on a Scottish trend as part of our spooky traditions isn't that unusual. Halloween traditions like carving pumpkins came from Celtic immigrants, too (per Mental Floss). But in other parts of the world, ghosts don't say "boo" at all. Instead, Slate reports, they opt for words like "hou" or "baf."
