The Collaborative International Dictionary of English v.0.48:

doofus \doo"fus\, n.
   a person who is stupid or inept; -- used contemptuously.
   [Also spelled dufus.] [Slang]

   Syn: dingdong, ding-a-ling. [PJC]

WordNet (r) 3.0 (2006):

doofus
    n 1: a stupid incompetent person [syn: dimwit, nitwit,
         half-wit, doofus]