The Collaborative International Dictionary of English v.0.48:

Unthinking \Un*think"ing\, a.
   1. Not thinking; not heedful; thoughtless; inconsiderate; as,
      unthinking youth.
      [1913 Webster]

   2. Not indicating thought or reflection; thoughtless.
      [1913 Webster]

            With earnest eyes, and round unthinking face,
            He first the snuffbox opened, then the case. --Pope.
      [1913 Webster] -- Un*think"ing*ly, adv. --
      Un*think"ing*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unthinking
    adv 1: in a thoughtless manner; "he stared thoughtlessly at the
           picture" [syn: thoughtlessly, unthinkingly,
           unthinking] [ant: thoughtfully]
    adj 1: not exhibiting or characterized by careful thought [syn:
           unreflective, unthinking, unthoughtful]
    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]
    3: mentally sluggish [syn: lumpish, lumpen, unthinking]

Moby Thesaurus II by Grady Ward, 1.0:

163 Moby Thesaurus words for "unthinking":
   airy, arbitrary, automatic, blank, blankminded, blind, brainless,
   calm, capricious, careless, casual, compulsive, conditioned, crazy,
   cursory, degage, discourteous, disregardant, disregardful,
   easygoing, empty, empty-headed, fatuous, feckless, flippant,
   foolish, forced, forgetful, free and easy, hasty, headless,
   heedless, idiotic, ignorant, ill-advised, ill-considered,
   ill-contrived, ill-devised, ill-gauged, ill-judged, illogical,
   imbecilic, impolite, impolitic, imprudent, impulsive, inadvertent,
   inadvisable, inane, incogitant, inconsiderate, indifferent,
   indiscreet, inept, inexpedient, injudicious, insane, insensate,
   insouciant, instinctive, involuntary, irrational, lackbrained,
   lazy, lean-minded, lean-witted, mad, mechanical, mindless,
   misadvised, misguided, moronic, myopic, neglectful, nirvanic,
   nonsensical, not bright, oblivious, of little brain, offhand,
   passive, pea-brained, perfunctory, pin-brained, quietistic, rash,
   reasonless, reckless, reflex, reflexive, regardless, relaxed,
   respectless, rude, senseless, short-sighted, shortsighted, silly,
   slackminded, slackwitted, stupid, tactless, thoughtfree,
   thoughtless, tranquil, unaccommodating, unadvised, uncalculating,
   uncaring, uncivil, uncomplaisant, unconscious, unconsidered,
   uncooperative, undesigned, undiplomatic, undiscerning,
   undiscriminating, unforeseeing, ungifted, unguarded, unguided,
   unheedful, unheeding, unhelpful, unideaed, unintellectual,
   unintelligent, unintended, unintentional, unmeant, unmeditated,
   unmindful, unobliging, unoccupied, unperceiving, unperceptive,
   unplanned, unpremeditated, unprepared, unprompted, unpurposed,
   unready, unreasonable, unreasoning, unreflecting, unreflective,
   unseeing, unsensible, unsolicitous, unsound, untactful, untalented,
   unthought-of, unthoughtful, unwilled, unwilling, unwise, unwitting,
   vacant, vacuous, witless, wrecking