Where did the expression “boo” come from? And why is this word associated with ghosts? :rolleyes: Yeah, like that scares me, too. Why not just jump out behind me and yell “gark,” it’ll work just as well.
As a WAG, I’d say that maybe it has to do with crying, a la boohoo.
But then I’m frequently wrong, and an idiot to booooooot. 
Far be it from me to disagree with Sue…She has my favorite handle on the whole board…
In the 1950’s and earlier it meant marijuana!
As an interjection, it is cited first in the 15th century. It not only was used to scare you but “used to express contempt or disapproval or to startle or frighten.” From my Webster’s Collegiate.