The Collaborative International Dictionary of English v.0.48:

Mindless \Mind"less\, a.
   1. Not indued with mind or intellectual powers; stupid;
      unthinking.
      [1913 Webster]

   2. Unmindful; inattentive; heedless; careless.
      [1913 Webster]

            Cursed Athens, mindless of thy worth. --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mindless
    adj 1: lacking the thinking capacity characteristic of a
           conscious being; "the shrieking of the mindless wind"
    2: requiring little mental effort; "mindless tasks"
    3: not mindful or attentive; "while thus unmindful of his steps
       he stumbled"- G.B.Shaw [syn: unmindful, forgetful,
       mindless] [ant: aware, mindful]
    4: devoid of intelligence [syn: asinine, fatuous, inane,
       mindless, vacuous]
    5: not marked by the use of reason; "mindless violence";
       "reasonless hostility"; "a senseless act" [syn: mindless,
       reasonless, senseless]

Moby Thesaurus II by Grady Ward, 1.0:

226 Moby Thesaurus words for "mindless":
   a stranger to, aimless, airy, aloof, anarchic, angry, animal,
   animalian, animalic, animalistic, apathetic, asinine, asleep,
   ataractic, beastlike, beastly, bestial, blase, blind to,
   blustering, blusterous, blustery, bored, brainless, brutal, brute,
   brutelike, brutish, careless, casual, caught napping, causeless,
   chaotic, cursory, dead to, deaf to, degage, designless, detached,
   devil-may-care, disinterested, dispassionate, disregardant,
   disregardful, distant, driftless, dumb, dysteleological, easygoing,
   fatuous, flippant, foolish, forgetful, frantic, free and easy,
   frenzied, fruity, furious, haphazard, headless, heedless, hellish,
   idiotic, ignorant, ill-advised, ill-considered, ill-contrived,
   ill-devised, ill-gauged, ill-judged, imbecilic, impassive,
   impercipient, impolitic, imprudent, in ignorance of, inadvisable,
   inattentive, incognizant, inconsiderate, incurious, indifferent,
   indiscreet, indiscriminate, inept, inexcitable, inexpedient,
   inexplicable, infuriate, injudicious, insensate, insensible,
   insensible to, insouciant, instinctive, instinctual, irrational,
   lackadaisical, lackbrained, lazy, lean-minded, lean-witted,
   listless, lunatic, mad, maniac, mental, misadvised, misguided,
   moronic, myopic, napping, negligent, nitwitted, non compos mentis,
   nonchalant, nonconceiving, nonrational, not bright, not with it,
   nuts, oblivious, obtuse, of little brain, offhand, orgasmic,
   orgastic, out of it, pandemoniac, pea-brained, perfunctory,
   phlegmatic, pin-brained, pococurante, promiscuous, purposeless,
   raging, random, ravening, raving, reasonless, reckless, regardless,
   respectless, rip-roaring, senseless, shortsighted, silly,
   slackminded, slackwitted, stochastic, stolid, storming, stormy,
   stray, stupid, subhuman, tactless, tempestuous, thick, thickheaded,
   thoughtless, troublous, tumultuous, turbulent, turned-off,
   unaccommodating, unaccountable, unadvised, unanxious, unaware,
   unaware of, uncomplaisant, unconcerned, unconscious,
   unconscious of, unconsidered, uncooperative, undiplomatic,
   undirected, undiscerning, undiscriminating, unforeseeing, ungifted,
   unhearing, unheedful, unheeding, unhelpful, uninquiring,
   uninsightful, unintellectual, unintelligent, uninterested,
   uninvolved, unknowing, unmindful, unmindful of, unmotivated,
   unobliging, unperceiving, unprehensive, unprepared, unready,
   unrealizing, unreasonable, unreasoning, unreflecting, unreflective,
   unseeing, unsensible, unsolicitous, unsound, unsuspecting,
   untactful, untalented, unthinking, unthoughtful, unwise, unwitting,
   uproarious, wild, withdrawn, witless, zoic, zooidal, zoologic