WordNet (r) 3.0 (2006):

lunkhead
    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]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "lunkhead":
   addlebrain, addlehead, addlepate, beefhead, blockhead, blubberhead,
   blunderhead, bonehead, boob, booby, bufflehead, cabbagehead,
   chowderhead, chucklehead, chump, clodhead, clodpate, clodpoll,
   dolt, dolthead, dullhead, dumbhead, dunderhead, dunderpate,
   fathead, goof, goon, jolterhead, jughead, knucklehead, meathead,
   muddlehead, mushhead, muttonhead, noodlehead, numskull, oaf,
   peabrain, pinbrain, pinhead, puddinghead, pumpkin head, puzzlehead,
   stupidhead, thickhead, thickskull, tottyhead