World War II aviator slang for "unidentified aircraft, presumably hostile," probably ultimately from
bogge, a variant of M.E.
bugge "a frightening specter" (see
bug). This was the presumed source of many dialect words, 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.1505 and popularized c.1800 in Eng. literature by Scott, Burns, etc.