The Collaborative International Dictionary of English v.0.48:

fuckhead \fuckhead\ n.
   1. a stupid person; -- disparaging and vulgar. [disparaging,
      vulgar slang]

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

WordNet (r) 3.0 (2006):

fuckhead
    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]