A boogieman (often spelled “bogeyman”) is an imaginary monster of indefinite appearance used to scare or threaten children. Boogiemen are known for hiding under beds or in closets, and for targeting children who disobey their parents.
“Bogey” is derived from Middle English bogge/bugge (16th century, “a frightening spectre”) and also shares ancestry with other European languages: bögge, böggel-mann (German); pwca, bwga (Welsh); etc. The earliest modern form appears to be the Scottish “bogle” or “ghost”, attested from circa 1500, and popularized in English literature much later, circa 1800, by Scott, Burns and others.
Whatever its source, “bogey” has spawned a range of other English meanings, including “dried nasal mucous” (aka “booger”), and “a golf score one over par.” This last usage supposedly dates back to 1890, when a player unable to do better than par for any hole declared that he “must be playing against the bogey-man.”
There is no evidence whatsoever of the existence of an actual bogeyman. This is one claim for which evidence is generally not put forward.