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]