Why do we use the word "boo" when we are displeased with something, and also when we want to scare someone?
Boo, including variations such as "bo" or "boh," goes back around 500 years.
It seems to be Scottish in origin, but some trace it back to a Greek word that meant "to cry aloud, roar or shout." And some say boo imitates the sound of a sad calf.
Booing as a sound of discontent seems to come in to use in the 18th century. Italian opera fans were said to be voracious boo birds.
Why is it such a useful word for scaring someone or expressing displeasure?
This is what I learned at slate.com: "The combination of the voiced, plosive b- and the roaring -oo sounds makes boo a particularly startling word. Some linguists argue that the "ooh" or "oh" sounds can be pronounced at a higher volume than other vowel sounds, such as the "ee" in 'wheel.' Since boo is a monosyllable, it can also be said very quickly, which may add to its scariness."
In Spain, by the way, you say "uuh" to scare somebody. In France it's "hou." And in Czech it's "baf."
Boo has also been used in the past as another name for marijuana. In modern slang it can mean a male or female friend.
And you might hear someone describe as person who is very timid as "afraid to say boo to a goose."
The idea is that if you were set upon by a flock of hissing geese and couldn't even summon the courage to say boo to them you would indeed by kind of a 'fraidy cat.