Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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