The Collaborative International Dictionary of English v.0.48:

Hard-hearted \Hard"-heart`ed\ (-h[aum]rt`[e^]d), a.
   Unsympathetic; inexorable; cruel; pitiless. --
   Hard"-heart`ed*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hardhearted
    adj 1: lacking in feeling or pity or warmth [syn: hardhearted,
           heartless] [ant: soft-boiled, softhearted]
    2: devoid of feeling for others; "an unfeeling wretch" [syn:
       hardhearted, stonyhearted, unfeeling]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "hardhearted":
   Philistine, brazen, callous, calloused, case-hardened, cold,
   cold of heart, coldblooded, coldhearted, conscienceless,
   flinthearted, flinty, hard, hard of heart, hard-boiled, hardened,
   heartless, impervious, indurated, insensitive, inured,
   lost to shame, obdurate, pachydermatous, proof against, seared,
   shameless, steeled against, steely, stony, stonyhearted,
   thick-skinned, unblushing, uncompassionate, unemotional, unfeeling,
   unmerciful, unnatural, unresponsive, unsympathetic