The Collaborative International Dictionary of English v.0.48:

callousness \cal"lous*ness\ n.
   lack of passion or feeling.

   Syn: unfeelingness, hardness, insensibility.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

callousness
    n 1: devoid of passion or feeling; hardheartedness [syn:
         unfeelingness, callousness, callosity, hardness,
         insensibility]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "callousness":
   Philistinism, analgesia, anesthesia, armor, callosity, callus,
   coldbloodedness, coldheartedness, coldness, deadness, density,
   dullness, durity, electronarcosis, flintiness, formidable defenses,
   hard heart, hard shell, hardenedness, hardheartedness, hardness,
   hardness of heart, heart of stone, heartlessness, impassibility,
   impenetrability, impenitence, impenitentness, imperception,
   imperceptiveness, imperceptivity, impercipience, imperviousness,
   inconsiderateness, induration, insensibility, insensibleness,
   insensitiveness, insensitivity, insentience, insolence, inuredness,
   irrepentance, narcosis, narcotization, nonrepentance, numbness,
   obduracy, obdurateness, obtuseness, pins and needles, pitilessness,
   resistance, restiveness, rhinoceros hide, seared conscience,
   solidity, steeliness, stoniness, thick skin, toughness,
   unabjectness, uncontriteness, unfeeling, unfeelingness,
   unmercifulness, unnaturalness, unperceptiveness,
   unresponsiveness