Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. Hardened in mind; insensible; unfeeling; unsusceptible.
      ``The callous diplomatist.'' --Macaulay.

            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. -- {Cal"lous*ness}, n.

              A callousness and numbness of soul. --Bentley.

Source : WordNet®

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