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]