Everyone knows that ghosts say “boo,” but when did they first start using that scary word? And what about ghosts in other parts of the world—do they have their own version of 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.
Variations of the word boo—including bo and boh—have been found in books published as far back as 500 years ago. While the Oxford English Dictionary notes the similarity between bo and the Latin boāre and the Greek βοãv, both meaning “to cry aloud, roar, shout,” it’s unlikely that bo and boo—as nonsensical exclamations—derived from these words. An etymological dictionary of Scottish from 1808 notes that the sound might denote "a sound in imitation of the cry of a calf," or be related to menacing creatures like the bu-kow and the bu-man (a possible ancestor of the modern bogeyman).
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.
If you want to frighten someone in Spain, you can say uuh (pronounced like ooh in English), and in France you can say hou. A Czech ghost might say baf. In most European languages, including non-Romance languages like Polish, the sound boo is also understood as an attempt to scare someone, but it comes in different spellings.* For example, the Spanish version is written as ¡bú!
The use of the word boo for jeering doesn’t seem to have come about until the 19th century. Boo is now used in slang to mean boyfriend or girlfriend (the term appears to derive from beau, meaning lover), and in the 1950s it was hipster slang for marijuana—but these usages seem to be unrelated.
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Explainer thanks Ben Zimmer of the Visual Thesaurus and Jesse Sheidlower of the Oxford English Dictionary.
Clarification, Oct. 31, 2011: This article originally listed Turkish along with Polish as an example of a non-Romance European language. Turkish may be spoken in parts of Europe, but it is not an Indo-European language. (Return to the revised sentence.)