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®

thoughtlessly
     adv 1: in a thoughtless manner; "he stared thoughtlessly at the
            picture" [syn: {unthinkingly}, {unthinking}] [ant: {thoughtfully}]
     2: showing thoughtlessness; "he treated his parents
        thoughtlessly" [ant: {thoughtfully}]