The Collaborative International Dictionary of English v.0.48:

Unfeeling \Un*feel"ing\, a.
   1. Destitute of feeling; void of sensibility; insensible;
      insensate.
      [1913 Webster]

   2. Without kind feelings; cruel; hard-hearted.
      [1913 Webster]

            To each his sufferings: all are men,
            Condemned alike to groan;
            The tender for another's pain,
            Th' unfeeling for his own.            --Gray.
      [1913 Webster] -- Un*feel"ing*ly, adv. --
      Un*feel"ing*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unfeeling
    adj 1: devoid of feeling for others; "an unfeeling wretch" [syn:
           hardhearted, stonyhearted, unfeeling]
    2: devoid of feeling or sensation; "unfeeling trees"

Moby Thesaurus II by Grady Ward, 1.0:

167 Moby Thesaurus words for "unfeeling":
   abiotic, affectless, analgesia, anesthesia, anesthetized, arctic,
   asleep, autistic, azoic, benumbed, blunt, bowelless, brutal,
   callosity, callous, calloused, callousness, cantankerous,
   catatonic, chill, chilly, churlish, cold, cold as charity,
   cold of heart, cold-blooded, coldblooded, coldbloodedness,
   coldhearted, coldheartedness, coldness, cool, crotchety, cruel,
   dead, deadened, deadness, dispassionate, dog-eat-dog, drugged,
   dull, dullness, dumb, electronarcosis, emotionally dead,
   emotionless, exacting, exanimate, flinthearted, flinty, frigid,
   frosted, frosty, frozen, hard, hard of heart, hard-boiled,
   hardened, hardhearted, hardheartedness, hardness,
   hardness of heart, harsh, heart of stone, heartless, heartlessness,
   icy, immovable, impassibility, impassible, impassive, imperception,
   imperceptive, imperceptiveness, imperceptivity, impercipience,
   impercipient, inanimate, inanimated, inclement, inconsiderateness,
   indurated, inert, inexcitable, inexorable, insensate,
   insensibility, insensible, insensibleness, insensitive,
   insensitiveness, insensitivity, insentience, insentient,
   insusceptible, ironhearted, lifeless, marblehearted, merciless,
   mute, narcosis, narcotization, nonconscious, nonemotional,
   nonliving, numb, numbed, numbness, obduracy, obdurate, objective,
   obtuse, obtuseness, out of touch, passionless, pins and needles,
   pitiless, pitilessness, relentless, remorseless, ruthless,
   self-absorbed, senseless, severe, soulless, spiritless, stony,
   stonyhearted, surly, thick skin, thick-skinned, thick-witted,
   tough, unaffectionate, unamiable, unanimated, uncompassionate,
   uncompassioned, unconscious, uncordial, unemotional, unfeelingness,
   unfelt, unforgiving, unimpassioned, unimpressionable, unkind,
   unloving, unmerciful, unmercifulness, unnatural, unnaturalness,
   unpassionate, unperceptive, unperceptiveness, unpitiful, unpitying,
   unremorseful, unresponding, unresponsive, unresponsiveness,
   unsusceptible, unsympathetic, unsympathizing, untouchable,
   unyielding, without mercy