Why do ghosts say “boo”?

"Ghosts were saying "boo!" by the middle of the 19th century, though the exclamation had been used to frighten English-speaking children for at least 100 years before that. Perhaps the first appearance of boo in print comes from the book-length polemic Scotch Presbyterian Eloquence Display’d (1738), in which author Gilbert Crokatt defines it as , “a word that’s used in the north of Scotland to frighten crying children.” (It's not clear why people in Scotland would want to frighten a crying child.) The verbal tactic had been adopted by proper ghosts—and people with sheets on their heads—by the 1820s at the latest." By Forrest Wickman /Slate Magazine / more