The Collaborative International Dictionary of English v.0.48:

Heartless \Heart"less\, a.
   1. Without a heart.
      [1913 Webster]

            You have left me heartless; mine is in your bosom.
                                                  --J. Webster.
      [1913 Webster]

   2. Destitute of courage; spiritless; despondent.
      [1913 Webster]

            Heartless they fought, and quitted soon their
            ground.                               --Dryden.
      [1913 Webster]

            Heartless and melancholy.             --W. Irwing.
      [1913 Webster]

   3. Destitute of feeling or affection; unsympathetic; cruel.
      "The heartless parasites." --Byron. -- Heart"less*ly,
      adv. -- Heart"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

heartless
    adj 1: lacking in feeling or pity or warmth [syn: hardhearted,
           heartless] [ant: soft-boiled, softhearted]
    2: devoid of courage or enthusiasm

Moby Thesaurus II by Grady Ward, 1.0:

179 Moby Thesaurus words for "heartless":
   Laodicean, Olympian, affectless, aloof, anesthetized, apathetic,
   arctic, autistic, benumbed, blah, blase, blunt, bowed-down,
   bowelless, brazen, brutal, callous, calloused, case-hardened,
   cast down, catatonic, chill, chilly, cold, cold as charity,
   cold of heart, cold-blooded, coldblooded, coldhearted, comatose,
   conscienceless, cool, cruel, dashed, dejected, depressed,
   desensitized, despairing, despondent, desponding, detached,
   discouraged, disheartened, disinterested, dispassionate,
   dispirited, dog-eat-dog, down, downcast, downhearted, drooping,
   droopy, drugged, dull, emotionally dead, emotionless, feeling low,
   flinthearted, flinty, frigid, frosted, frosty, frozen, gritless,
   gutless, hard, hard of heart, hard-boiled, hardened, hardhearted,
   harsh, hebetudinous, hopeless, hypochondriac, hypochondriacal, icy,
   immovable, impassible, impassive, in a stupor, in low spirits,
   in the depths, in the doldrums, in the dumps, inclement,
   indifferent, indurated, inexcitable, inexorable, inhuman, inhumane,
   insensitive, insouciant, insusceptible, inured, languid,
   languishing, lethargic, listless, lost to shame, low, low-spirited,
   merciless, nonchalant, nonemotional, numb, numbed, obdurate,
   objective, obtuse, out of touch, passionless, passive, pessimistic,
   phlegmatic, pining, pitiless, pluckless, relentless, remorseless,
   resigned, ruthless, seared, self-absorbed, shameless, slack,
   sluggish, soporific, soulless, spiritless, spunkless, stoic,
   stonyhearted, stupefied, subdued, suicidal, supine, torpid,
   unaffectionate, unblushing, uncaring, uncompassionate,
   uncompassioned, unconcerned, uncourageous, undaring, unemotional,
   unfeeling, unforgiving, ungallant, unheroic, unimpassioned,
   unimpressionable, uninterested, unintrepid, unkind, unloving,
   unmerciful, unnatural, unpassionate, unpitiful, unpitying,
   unremorseful, unresponding, unresponsive, unsoldierlike,
   unsoldierly, unsusceptible, unsympathetic, unsympathizing,
   untouchable, unvaliant, unvalorous, unyielding, weary of life,
   withdrawn, without mercy, woebegone, world-weary