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):

callously
    adv 1: in a callous way; "he callously exploited their feelings"
           [syn: callously, unfeelingly]