Why do ghosts say "Boo!"?

All my google-fu hasn’t been able to produce an answer to this question.

Why do we associate ghosts with the word “boo”?

I can’t even think of a pop-culture reference to a ghost saying “boo,” yet it’s a universally recognized meme. Pictures of ghosts are almost inevitably captioned “BOO!” and anyone who dresses up for Halloween as a ghost (by wearing a sheet with holes cut out for the eyes, of course) will say “boo” to whoever they meet. Ask anyone what a ghost says, and the answer will invariably be “boo”! But has a ghost in fiction or in reported fact ever actually said “boo”? Where does the idea come from?

I think it has to do with the sound of the letter “B” and the element of surprise. The letter “B” has a rather harsh, abrupt sound, so “boo” would startle someone more than, say, “foo” or “thoo.”

Just my WAG.

Certainly the ghosts in “Casper the Friendly Ghost” and “Spooky” said “boo” in those comics I read as a kid. That is a pop-culture reference.

I think “boo” is a word you say when you surprise someone wit your appearance. This goes all the way back to small babies and playing “peek-a-boo” where the “boo” part is usually emphasized in a high-pitched voice. So “boo” seems to be a word we associate with surprise. Why that is, I don’t know.

I googled “boo in other languages” and got this, which tells you that the sounds are similar in at least a few other languages. Maybe the answer lies in examining the sound of the phoneme and how it is produced by humans (with a an outthrust, rounded mouth). The Online Etymology Dictionary seems to think this might be the case, but handy-sounding etymological explanations are fraught with peril.

they don’t say “boo” in japanese. “peek-a-boo” over here is “i-nai-nai-BAA!”* which, as my brother says, is kind of advertising your scare (the BAA part) though I have scared one or two of my students in the halls just muttering i-nai-nai… BAAA as I turned to them. Got one to drop her books, muahahaha

*in Japan the “i” or イ has a sound like “ee”

It seemed odd to me as a child that the ghosts in American comics said “Boo!” like they were trying to surprise people.

British ghosts say “Woooo!” – more of a long, wailing, moaning sound. More melancholy, you know.

Gaijin smash.