Mythical "person/monster" used by parents to scare their children into obedience.
eg - "Now Johnny, if you don't clean your room, the Boogeyman is gonna get ya!"
Like
Santa Claus is used for
positive reinforcement for young children, The Boogeyman is used for
negative reinforcement.
Note - much more common in the first half of the 20th-century, rarely used now. Sometimes used by adults when attempting to describe an unknown force of
mayhem, like
Gremlins.
eg2 - "What happened to your car Mike? Looks like the Boogeyman beat it with an
ugly-stick!"