The Collaborative International Dictionary of English v.0.48:

Stupid \Stu"pid\, a. [L. stupidus, fr. stupere to be stupefied:
   cf. F. stupide.]
   1. Very dull; insensible; senseless; wanting in
      understanding; heavy; sluggish; in a state of stupor; --
      said of persons.
      [1913 Webster]

            O that men . . . should be so stupid grown . . .
            As to forsake the living God!         --Milton.
      [1913 Webster]

            With wild surprise,
            A moment stupid, motionless he stood. --Thomson.
      [1913 Webster]

   2. Resulting from, or evincing, stupidity; formed without
      skill or genius; dull; heavy; -- said of things.
      [1913 Webster]

            Observe what loads of stupid rhymes
            Oppress us in corrupted times.        --Swift.
      [1913 Webster]

   Syn: Simple; insensible; sluggish; senseless; doltish;
        sottish; dull; heavy; clodpated. -- Stu"pid*ly, adv.
        -- Stu"pid*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

stupid
    adj 1: lacking or marked by lack of intellectual acuity [ant:
           smart]
    2: in a state of mental numbness especially as resulting from
       shock; "he had a dazed expression on his face"; "lay
       semiconscious, stunned (or stupefied) by the blow"; "was
       stupid from fatigue" [syn: dazed, stunned, stupefied,
       stupid(p)]
    3: lacking intelligence; "a dull job with lazy and unintelligent
       co-workers" [syn: unintelligent, stupid] [ant:
       intelligent]
    n 1: a person who is not very bright; "The economy, stupid!"
         [syn: stupid, stupid person, stupe, dullard,
         dolt, pudding head, pudden-head, poor fish,
         pillock]

Moby Thesaurus II by Grady Ward, 1.0:

179 Moby Thesaurus words for "stupid":
   Boeotian, absurd, addled, apish, asinine, backward, balmy, barmy,
   batty, beef-brained, beef-witted, beetleheaded, befooled, beguiled,
   besotted, blear-eyed, blear-witted, block, blockheaded, blockish,
   boneheaded, bootless, boring, bovine, brainless, brute, brutish,
   buffoonish, chuckleheaded, chumpish, clod, cloddish, cockamamie,
   cockeyed, cowish, crass, crazy, credulous, cretinous, cuckoo,
   daffy, daft, dazed, dense, dim, dim-witted, dimwit, dizzy, dolt,
   doltish, donkey, dope, dopey, doting, dozy, dull, dull-witted,
   dullard, dullhead, dumb, dumb cluck, dumbbell, dummkopf, dummy,
   dunce, duncical, duncish, empty-headed, fat, fatheaded, fatuitous,
   fatuous, flaky, fond, fool, foolhardy, foolheaded, foolish,
   frivolous, fuddled, futile, gaga, goofy, goosey, gowk, gross,
   gulled, half-assed, half-baked, half-witted, ho-hum, humdrum,
   idiot, idiotic, ignoramus, ill-advised, imbecile, imbecilic, inane,
   ineducable, inept, infatuated, insane, insipid, irrational,
   irresponsible, jerky, jobbernowl, klutzy, kooky, lackwit,
   lamebrain, laughable, lightweight, looby, loon, loony, ludicrous,
   lumbering, lumpish, mad, maudlin, mindless, monotonous, moron,
   moronic, niais, nincompoop, ninny, ninnyhammer, nitwit, noddy,
   nonsensical, numskulled, nutty, oafish, obtuse, opaque, pinhead,
   prosaic, put, retarded, ridiculous, risible, sappy, scatterbrained,
   screwy, senseless, sentimental, silly, simple, simpleton, slow,
   slow-witted, sluggish, sottish, stolid, subnormal, tedious, thick,
   thick-witted, thickheaded, thickwit, thoughtless, tiresome, torpid,
   unimaginative, uninspired, unintelligent, uninteresting,
   unteachable, vacuous, vapid, wacky, weak-minded, wet, witless,
   witling, wrongheaded