The Collaborative International Dictionary of English v.0.48:

Dunce \Dunce\, n. [From Joannes Duns Scotus, a schoolman called
   the Subtle Doctor, who died in 1308. Originally in the phrase
   "a Duns man". See Note below.]
   One backward in book learning; a child or other person dull
   or weak in intellect; a dullard; a dolt.
   [1913 Webster]

         I never knew this town without dunces of figure.
                                                  --Swift.
   [1913 Webster]

   Note: The schoolmen were often called, after their great
         leader Duns Scotus, Dunsmen or Duncemen. In the revival
         of learning they were violently opposed to classical
         studies; hence, the name of Dunce was applied with
         scorn and contempt to an opposer of learning, or to one
         slow at learning, a dullard.
         [1913 Webster]

WordNet (r) 3.0 (2006):

dunce
    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]