The Collaborative International Dictionary of English v.0.48:

Uncompassionate \Uncompassionate\
   See compassionate.

WordNet (r) 3.0 (2006):

uncompassionate
    adj 1: lacking compassion or feeling for others; "nor silver-
           shedding tears could penetrate her uncompassionate sire"-
           Shakespeare [ant: compassionate]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "uncompassionate":
   bowelless, callous, coldhearted, cruel, disagreeable, dog-eat-dog,
   flinty, hard, hard-boiled, hardhearted, harsh, heartless, ill,
   inclement, inexorable, inhospitable, merciless, obdurate, pitiless,
   relentless, remorseless, ruthless, unaffectionate, unamiable,
   unbenign, unbenignant, uncompassioned, uncordial, unemotional,
   unfeeling, unforgiving, ungenial, ungracious, unkind, unkindly,
   unloving, unmerciful, unpitiful, unpitying, unremorseful,
   unsympathetic, unsympathizing, unyielding, without mercy