The Collaborative International Dictionary of English v.0.48:

Witless \Wit"less\, a.
   Destitute of wit or understanding; wanting thought; hence,
   indiscreet; not under the guidance of judgment. "Witless
   bravery." --Shak.
   [1913 Webster]

         A witty mother! witless else her son.    --Shak.
   [1913 Webster]

         Witless pity breedeth fruitless love.    --Fairfax.
   [1913 Webster] -- Wit"less*ly, adv. -- Wit"less*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

witless
    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:

195 Moby Thesaurus words for "witless":
   a stranger to, abnormal, apish, asinine, asleep, batty, bedlamite,
   befooled, beguiled, bereft of reason, besotted, blind to,
   brainless, brainsick, buffoonish, caught napping, cockeyed,
   crackbrained, cracked, crazed, crazy, credulous, daffy, daft,
   dazed, dead to, deaf to, deluded, demented, deprived of reason,
   deranged, disoriented, distraught, dizzy, doting, dumb,
   empty-headed, fatuitous, fatuous, featherbrained, featherheaded,
   flaky, flighty, fluttery, fond, fool, foolheaded, foolish,
   frivolous, fuddled, futile, gaga, giddy, giddy-brained,
   giddy-headed, giddy-pated, giddy-witted, goofy, gulled,
   hallucinated, harebrain, harebrained, headless, idiotic, ignorant,
   ill-advised, ill-considered, ill-contrived, ill-devised,
   ill-gauged, ill-judged, imbecile, impercipient, impolitic,
   imprudent, in ignorance of, inadvisable, inane, incognizant,
   inconsiderate, indiscreet, inept, inexpedient, infatuated,
   injudicious, insane, insensate, insensible, insensible to,
   irrational, kooky, lackbrained, lean-minded, lean-witted, loco,
   loony, lunatic, mad, maddened, manic, maudlin, mazed, mental,
   mentally deficient, meshuggah, mindless, misadvised, misguided,
   moon-struck, moronic, myopic, napping, nitwitted, non compos,
   non compos mentis, nonconceiving, not all there, not bright,
   not right, not with it, nutty, odd, of little brain,
   of unsound mind, off, out of it, pea-brained, pin-brained, psycho,
   queer, rattlebrained, rattleheaded, rattlepated, reasonless,
   reckless, sappy, scatterbrained, scramblebrained, screwy,
   senseless, sentimental, shatterbrained, shortsighted, sick, silly,
   slackminded, slackwitted, stark-mad, stark-staring mad, strange,
   stupid, tetched, thoughtless, touched, unadvised, unaware,
   unaware of, unbalanced, unconscious, unconscious of, unconsidered,
   undiscerning, unforeseeing, ungifted, unhearing, unhinged,
   uninsightful, unintellectual, unintelligent, unknowing, unmindful,
   unmindful of, unperceiving, unprehensive, unrealizing,
   unreasonable, unreasoning, unreflecting, unreflective, unsane,
   unseeing, unsensible, unsettled, unsound, unsuspecting, untalented,
   unthinking, unthoughtful, unwise, unwitting, unwitty, wacky,
   wandering, weak-minded, wet