Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

            With wild surprise, A moment stupid, motionless he
            stood.                                --Thomson.

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

            Observe what loads of stupid rhymes Oppress us in
            corrupted times.                      --Swift.

   Syn: Simple; insensible; sluggish; senseless; doltish;
        sottish; dull; heavy; clodpated. -- {Stu"pid*ly}, adv.
        -- {Stu"pid*ness}, n.

Source : WordNet®

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: without much intelligence; "a dull job with lazy and
        unintelligent co-workers" [syn: {unintelligent}] [ant: {intelligent}]

stupid
     n : a person who is not very bright; "The economy, stupid!"
         [syn: {stupid person}, {dullard}, {dolt}, {pudding head},
          {pudden-head}, {poor fish}, {pillock}]