The Collaborative International Dictionary of English v.0.48:

bonehead \bone"head`\ n.
   a person of low intelligence; a dunce; a blockhead; -- used
   deprecatingly to express a low opinion of someone's
   intelligence or capabilities.

   Syn: dunce, dunderhead, numskull, blockhead, lunkhead,
        hammerhead, knucklehead, loggerhead, muttonhead,
        shithead, fuckhead.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

bonehead
    n 1: a stupid person; these words are used to express a low
         opinion of someone's intelligence [syn: dunce,
         dunderhead, numskull, blockhead, bonehead,
         lunkhead, hammerhead, knucklehead, loggerhead,
         muttonhead, shithead, dumbass, fuckhead]