WordNet (r) 3.0 (2006):

nitwitted
    adj 1: (of especially persons) lacking sense or understanding or
           judgment [syn: nitwitted, senseless, soft-witted,
           witless]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "nitwitted":
   beetleheaded, blockheaded, blunderheaded, boneheaded, brainless,
   cabbageheaded, chowderheaded, chuckleheaded, clodpated, dumbheaded,
   dunderheaded, fatheaded, foolish, jolterheaded, joltheaded,
   knuckleheaded, lunkheaded, mindless, muttonheaded, numskulled,
   pumpkin-headed, sapheaded, senseless, silly, stupidheaded, unwitty,
   weak-minded, witless, woodenheaded