Source : Webster's Revised Unabridged Dictionary (1913)

Heartless \Heart"less\, a.
   1. Without a heart.

            You have left me heartess; mine is in your bosom.
                                                  --J. Webster.

   2. Destitute of courage; spiritless; despodent.

            Heartless they fought, and quitted soon their
            ground.                               --Dryden.

            Heartless and melancholy.             --W. Irwing.

   3. Destitute of feeling or affection; unsympathetic; cruel.
      ``The heartless parasites.'' --Byron. -- {Heart"less*ly},
      adv. -- {Heart"less*ness}, n.

Source : WordNet®

heartless
     adj 1: marked by lack of heart or feeling; "a heartless tyrant";
            "heartless words" [ant: {hearted}]
     2: lacking in feeling or pity or warmth [syn: {hardhearted}]
        [ant: {softhearted}]
     3: devoid of courage or enthusiasm