The Collaborative International Dictionary of English v.0.48:

Boogeyman \Boog"ey*man\, Bogeyman \Bo"gey*man\, n.
   Something frightful, as a specter; anything imaginary that
   causes needless fright; something used to excite needless
   fear; also, something really dangerous, or an imaginary
   monster, used to frighten children, etc. "Go to sleep or the
   Boogeyman will get you."

   Syn: Hobgoblin; goblin; specter; ogre; scarecrow; booger;
        bugaboo; bugbear.
        [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

boogeyman
    n 1: an imaginary monster used to frighten children [syn:
         bogeyman, bugbear, bugaboo, boogeyman, booger]