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):

thoughtlessness
    n 1: the quality of failing to be considerate of others [syn:
         inconsideration, inconsiderateness, thoughtlessness]
         [ant: considerateness, consideration, thoughtfulness]
    2: the trait of not thinking carefully before acting [syn:
       unthoughtfulness, thoughtlessness] [ant:
       thoughtfulness]

Moby Thesaurus II by Grady Ward, 1.0:

166 Moby Thesaurus words for "thoughtlessness":
   abandon, absentmindedness, asininity, battiness, brainlessness,
   buffoonery, callowness, caprice, careless abandon, carelessness,
   casualness, childishness, clownishness, crackpottedness,
   crankiness, craziness, cursoriness, daffiness, desipience,
   disregard, disregardfulness, distraction, easiness, eccentricity,
   empty-headedness, fatuity, fatuousness, fecklessness, flightiness,
   flippancy, folly, foolery, foolheadedness, foolishness,
   forgetfulness, frivolity, frivolousness, giddiness, goofiness,
   hastiness, heedlessness, idiocy, imbecility, immaturity,
   improvidence, imprudence, inability, inadeptness, inadequacy,
   inadvertence, inadvertency, inadvisability, inanity, inaptitude,
   inaptness, inattention, inattentiveness, incapability, incapacity,
   incompetence, incompetency, inconsiderateness, inconsideration,
   incuriosity, indexterity, indifference, indiscreetness,
   indiscretion, ineffectiveness, ineffectuality, inefficiency,
   ineptitude, ineptness, inexpedience, injudiciousness, inobservance,
   insanity, insouciance, irrationality, lack of foresight,
   lax stewardship, laziness, levity, lightmindedness, lunacy,
   madness, maladjustment, maladroitness, mediocrity, mercuriality,
   mindlessness, negligence, niaiserie, nonobservance, nugacity,
   nuttiness, oblivion, obliviousness, offhandedness, pedestrianism,
   perfunctoriness, pompousness, poor husbandry, puerility, queerness,
   reasonlessness, recklessness, regardlessness, rustiness, sappiness,
   screwiness, senselessness, shallowness, shiftlessness, silliness,
   skill-lessness, sprezzatura, stuffiness, stupidity, superficiality,
   tactlessness, thriftlessness, triflingness, triviality,
   unaccommodatingness, unalertness, unaptness, unawareness,
   uncleverness, unconsciousness, undeftness, undexterousness,
   undextrousness, unfitness, unfittedness, unheedfulness,
   unhelpfulness, unintelligence, unintentiveness, unmindfulness,
   unobligingness, unobservance, unpreparedness, unproficiency,
   unreadiness, unreason, unreasonableness, unschooledness,
   unsensibleness, unskillfulness, unsolicitousness, unsolicitude,
   unsoundness, unthinkingness, unthoughtfulness, untrainedness,
   unwariness, unwatchfulness, unwisdom, unwiseness, volatility,
   wackiness, weirdness, witlessness, zaniness, zanyism