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®

thoughtless
     adj 1: showing lack of careful thought; "the debate turned into
            thoughtless bickering" [ant: {thoughtful}]
     2: marked by or paying little heed or attention; "We have
        always known that heedless self-interest was bad morals;
        we know now that it is bad economics"--Franklin D.
        Roosevelt; "heedless of danger"; "heedless of the child's
        crying" [syn: {heedless}, {unheeding}] [ant: {heedful}]
     3: 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: {uncaring}, {unthinking}]
     4: (usually followed by `of') without due thought or
        consideration; "careless of the consequences"; "the
        proverbial grasshopper--thoughtless of tomorrow";
        "crushing the blooms with regardless tread" [syn: {careless(p)},
         {thoughtless(p)}, {regardless}]