The Collaborative International Dictionary of English v.0.48:

Dunderhead \Dun"der*head`\, n. [Prov. Eng. also dunderpoll, from
   dunder, same as thunder.]
   A dunce; a numskull; a blockhead. --Beau. & Fl.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dunderhead
    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]