The Collaborative International Dictionary of English v.0.48:

Stupidity \Stu*pid"i*ty\, n. [L. stupiditas: cf. F.
   stupidit['e].]
   1. The quality or state of being stupid; extreme dullness of
      perception or understanding; insensibility; sluggishness.
      [1913 Webster]

   2. Stupor; astonishment; stupefaction. [R.]
      [1913 Webster]

            A stupidity
            Past admiration strikes me, joined with fear.
                                                  --Chapman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stupidity
    n 1: a poor ability to understand or to profit from experience
         [ant: intelligence]
    2: a stupid mistake [syn: stupidity, betise, folly,
       foolishness, imbecility]