The Collaborative International Dictionary of English v.0.48:

Unintelligent \Unintelligent\
   See intelligent.

WordNet (r) 3.0 (2006):

unintelligent
    adj 1: lacking intelligence; "a dull job with lazy and
           unintelligent co-workers" [syn: unintelligent,
           stupid] [ant: intelligent]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "unintelligent":
   artless, awkward, blankminded, brainless, callow, dumb, empty,
   empty-headed, fatuous, foolish, gauche, green, groping, half-assed,
   headless, ignorant, inadept, inane, inapt, inattentive,
   inefficient, inept, inexperienced, inexpert, innocent, insensate,
   irrational, know-nothing, lackbrained, lean-minded, lean-witted,
   mediocre, mindless, naive, nescient, not bright, of little brain,
   pea-brained, pedestrian, pin-brained, poor, raw, reasonless,
   senseless, simple, skill-less, slackminded, slackwitted,
   strange to, tentative, thoughtless, unacquainted, unapprized,
   unapt, uncomprehending, unconversant, undeft, undexterous,
   undextrous, unenlightened, unfacile, unfamiliar, ungifted,
   unilluminated, uninformed, uninitiated, unintellectual, unknowing,
   unposted, unproficient, unreasoning, unripe, unskillful, unsure,
   untalented, unthinking, unversed, unwise, vacuous, weak, witless