The Collaborative International Dictionary of English v.0.48:

Thoughtless \Thought"less\, adv.
   1. Lacking thought; careless; inconsiderate; rash; as, a
      thoughtless person, or act.
      [1913 Webster]

   2. Giddy; gay; dissipated. [R.] --Johnson.
      [1913 Webster]

   3. Deficient in reasoning power; stupid; dull.
      [1913 Webster]

            Thoughtless as monarch oaks that shade the plain.
                                                  --Dryden.
      [1913 Webster] -- Thought"less*ly, adv. --
      Thought"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thoughtless
    adj 1: showing lack of careful thought; "the debate turned into
           thoughtless bickering" [ant: thoughtful]
    2: without care or thought for others; "the thoughtless saying
       of a great princess on being informed that the people had no
       bread; `Let them eat cake'" [syn: thoughtless, uncaring,
       unthinking]

Moby Thesaurus II by Grady Ward, 1.0:

217 Moby Thesaurus words for "thoughtless":
   absent-minded, airy, apish, arbitrary, artless, asinine, batty,
   befooled, beguiled, besotted, blank, blankminded, brainless, brash,
   buffoonish, calm, capricious, careless, casual, cockeyed, crazy,
   credulous, cursory, daffy, daft, dazed, degage, discourteous,
   disregardant, disregardful, distracted, distrait, dizzy, doting,
   dumb, easygoing, empty, empty-headed, fatuitous, fatuous,
   featherbrained, featherheaded, feckless, flaky, flippant, fluttery,
   fond, fool, foolheaded, foolish, forgetful, free and easy,
   frivolous, fuddled, futile, gaga, giddy, giddy-brained,
   giddy-headed, giddy-pated, giddy-witted, goofy, grasshopper,
   gulled, half-assed, happy-go-lucky, harebrain, harebrained, hasty,
   heedless, hotheaded, idiotic, ill-advised, ill-considered,
   ill-contrived, ill-devised, ill-gauged, ill-judged, imbecile,
   impolite, impolitic, improvident, imprudent, inadept, inadvertent,
   inadvisable, inane, inapt, inattentive, incautious, incogitant,
   inconsiderate, incurious, indifferent, indiscreet, inefficient,
   inept, inexpedient, inexpert, infatuated, injudicious, inobservant,
   insane, insensate, insensitive, insouciant, irrational, kooky,
   lazy, loony, mad, madcap, maudlin, mediocre, mindless, misadvised,
   misguided, moronic, myopic, neglectful, negligent, nirvanic, nutty,
   oblivious, offhand, passive, pedestrian, perfunctory, poor,
   quietistic, rash, rattlebrained, rattleheaded, rattlepated,
   reasonless, reckless, regardless, relaxed, remiss, respectless,
   rude, sappy, scatterbrained, scramblebrained, screwy, selfish,
   senseless, sentimental, sharp, shatterbrained, shiftless,
   shortsighted, silly, skill-less, stupid, tactless, thoughtfree,
   thriftless, tranquil, unaccommodating, unadvised, unapt,
   uncalculating, uncaring, unceremonious, uncomplaisant,
   unconsidered, uncooperative, undeft, undexterous, undextrous,
   undiplomatic, undiscerning, uneconomical, unfacile, unforeseeing,
   ungracious, unguarded, unheedful, unheeding, unhelpful, unideaed,
   unintellectual, unintelligent, unmarking, unmindful, unnoticing,
   unnoting, unobliging, unobservant, unobserving, unoccupied,
   unprepared, unproficient, unproviding, unready, unreasonable,
   unreasoning, unreflecting, unreflective, unremarking, unseeing,
   unsensible, unskillful, unsolicitous, unsound, untactful,
   unthinking, unthoughtful, unthrifty, unwise, vacant, vacuous,
   wacky, wet, witless, wrecking