Etymology
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boggart (n.)
also boggard, specter, goblin, sprite," especially one supposed to haunt a particular spot, 1560s; see bug (n.).
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boggle (v.)
1590s, "to start with fright (as a startled horse does), shy, take alarm," from Middle English bugge "specter" (among other things, supposed to scare horses at night); see bug (n.); also compare bogey (n.1), boggart. The meaning " hesitate, stop as if afraid to proceed in fear of unforeseen difficulties" is from 1630s; that of "confound, cause to hesitate" is from 1640s. As a noun from 1650s. Related: Boggled; boggling; boggler (from c. 1600 as "one who hesitates").
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poltergeist (n.)

"a noisy spirit, a ghost which makes its presence known by noises," 1838, from German Poltergeist, literally "noisy ghost," from poltern "make noise, rattle" (from PIE root *bhel- (4) "to sound, ring, roar;" source of bellow, bell) + Geist "ghost" (see ghost (n.)). In the native idiom of Northern England, such phenomena likely would be credited to a boggart.

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bogey (n.1)
World War II aviator slang for "unidentified aircraft, presumably hostile," probably ultimately from bogge, a variant of Middle English bugge "a frightening specter" (see bug (n.)).

Thus it shares ancestry with many dialect words for "ghost, specter," such as bog/bogge (attested 16c.-17c.), bogeyman (16c.), boggart "specter that haunts a gloomy spot" (c. 1570, in Westmoreland, Lancashire, Cheshire, and Yorkshire). The earliest modern form appears to be Scottish bogle "ghost," attested from c. 1500 and popularized c. 1800 in English literature by Scott, Burns, etc.
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