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]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "stupidity":
   absurdity, absurdness, act of folly, asininity, battiness,
   blockishness, blunder, boobishness, boorishness, bootlessness,
   botch, bovinity, brainlessness, buffoonery, bungle, cloddishness,
   clownishness, crackpottedness, crankiness, crassness, craziness,
   cretinism, crudeness, daffiness, denseness, density, desipience,
   dim-wittedness, dimness, doltishness, dull-wittedness, dullardism,
   dullness, dumb trick, dumbness, duncery, eccentricity,
   empty-headedness, fatuity, fatuousness, faux pas, folly, foolery,
   foolhardiness, foolheadedness, foolishness, frivolity,
   frivolousness, gaffe, giddiness, goofiness, grossness, hebetude,
   idiocy, imbecility, imprudence, inanity, indiscretion, ineptitude,
   insanity, irrationality, irresponsibility, lethargy, loutishness,
   ludicrousness, lumpishness, lunacy, madness, mindlessness,
   niaiserie, ninnyism, nonsense, nugacity, nuttiness, oafdom,
   oafishness, obtuseness, opacity, queerness, ridiculousness,
   risibility, sappiness, screwiness, senselessness, silliness,
   simpletonianism, simplicity, slowness, sluggishness, solecism,
   sottise, sottishness, stolidity, stupid thing, thick-headedness,
   thick-wittedness, thoughtlessness, triflingness, triviality,
   unteachability, unwise step, wackiness, weak-mindedness, weirdness,
   witlessness, wrongheadedness, yokelism, zaniness, zanyism



Bouvier's Law Dictionary, Revised 6th Ed (1856):

STUPIDITY, med. jur. That state of the mind which cannot perceive and
embrace the data presented to it by the senses; and therefore the stupid
person can, in general, form no correct judgment. It is a want of the
perceptive powers. Ray, Med. Jur. c. 3, Sec. 40. Vide Imbecility.