Why do ghosts say "Boo!"?

General Questions
#1

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?

#2

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.

#3

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.

#4

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.

#5

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”

#6

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.

#7

Gaijin smash.