The Collaborative International Dictionary of English v.0.48:

Callous \Cal"lous\, a. [L. callosus callous hard, fr. callum,
   callus, callous skin: cf. F. calleux.]
   1. Hardened; indurated. "A callous hand." --Goldsmith. "A
      callous ulcer." --Dunglison.
      [1913 Webster]

   2. Hardened in mind; insensible; unfeeling; unsusceptible.
      "The callous diplomatist." --Macaulay.
      [1913 Webster]

            It is an immense blessing to be perfectly callous to
            ridicule.                             --T. Arnold.

   Syn: Obdurate; hard; hardened; indurated; insensible;
        unfeeling; unsusceptible. See Obdurate. --
        Cal"lous*ly, adv.
        [1913 Webster]

              A callousness and numbness of soul. --Bentley.
        [1913 Webster]

WordNet (r) 3.0 (2006):

callous
    adj 1: emotionally hardened; "a callous indifference to
           suffering"; "cold-blooded and indurate to public opinion"
           [syn: callous, indurate, pachydermatous]
    2: having calluses; having skin made tough and thick through
       wear; "calloused skin"; "with a workman's callous hands"
       [syn: calloused, callous, thickened]
    v 1: make insensitive or callous; deaden feelings or morals
         [syn: callous, cauterize, cauterise]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "callous":
   Philistine, anneal, apathetic, asleep, backed, benumbed, brazen,
   brutalize, calcified, calcify, calloused, case harden,
   case-hardened, cold, cold of heart, coldblooded, coldhearted,
   conscienceless, cornify, crusted, crusty, crystallized, dead,
   deadened, dull, firm, flinthearted, flinty, fossilize, fossilized,
   granulated, hard, hard of heart, hard-boiled, hard-nosed, harden,
   hardened, hardhearted, heartless, hornified, impassible,
   impenitent, imperceptive, impercipient, impervious, incrusted,
   indifferent, indurate, indurated, insensate, insensible,
   insensitive, insentient, insolent, inure, inured, lapidified,
   lapidify, lithify, lost to shame, numb, numbed, obdurate, obtuse,
   ossified, ossify, pachydermatous, petrified, petrify,
   proof against, reinforced, rigidified, sclerotic, seared, set,
   shameless, solidified, steel, steeled, steeled against, steely,
   stiffened, stony, stonyhearted, strengthened, temper,
   thick-skinned, thick-witted, tough, toughen, toughened, unabject,
   unblushing, uncaring, uncompassionate, uncontrite, unemotional,
   unfeeling, unfelt, unmelted, unmerciful, unnatural, unperceptive,
   unrepentant, unrepenting, unresponsive, unsoftened, unsympathetic,
   untouched, vitrified, vitrify



The Devil's Dictionary (1881-1906):

CALLOUS, adj.  Gifted with great fortitude to bear the evils
afflicting another.
    When Zeno was told that one of his enemies was no more he was
observed to be deeply moved.  "What!" said one of his disciples, "you
weep at the death of an enemy?"  "Ah, 'tis true," replied the great
Stoic; "but you should see me smile at the death of a friend."