Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   3. Deficient in reasoning power; stupid; dull.

            Thoughtless as monarch oaks that shade the plain.
                                                  --Dryden.
      -- {Thought"less*ly}, adv. -- {Thought"less*ness}, n.

Source : WordNet®

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