The Collaborative International Dictionary of English v.0.48:

Merciless \Mer"ci*less\, a.
   Destitute of mercy; cruel; unsparing; -- said of animate
   beings, and also, figuratively, of things; as, a merciless
   tyrant; merciless waves.
   [1913 Webster]

         The foe is merciless, and will not pity. --Shak.
   [1913 Webster]

   Syn: Cruel; unmerciful; remorseless; ruthless; pitiless;
        barbarous; savage.
        [1913 Webster] -- Mer"ci*less*ly, adv. --
        Mer"ci*less*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

merciless
    adj 1: having or showing no mercy; "the merciless enemy"; "a
           merciless critic"; "gave him a merciless beating" [syn:
           merciless, unmerciful] [ant: merciful]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "merciless":
   atrocious, barbarian, barbaric, barbarous, bestial, bloody,
   bowelless, brutal, brutish, callous, cold, crude, cruel, cutthroat,
   dog-eat-dog, feral, ferine, ferocious, fierce, hard, harsh,
   heartless, illiberal, implacable, inclement, inconsiderate,
   indifferent, inexorable, inflexible, inhuman, inhumane,
   insensitive, kill-crazy, malevolent, malign, malignant, mortal,
   murderous, noncivilized, pitiless, relentless, remorseless, rough,
   rude, ruthless, sanguinary, savage, severe, tameless, thoughtless,
   tough, tyrannical, unappeasable, unbending, uncalled-for,
   uncharitable, uncivilized, uncompassionate, uncompassioned,
   unfeeling, unflinching, unforgiving, ungentle, ungracious,
   unmerciful, unmoved, unpitiful, unpitying, unrelenting,
   unremorseful, unsparing, unsympathetic, unsympathizing, untamed,
   unyielding, vicious, wanton, wild, without mercy