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]