The Collaborative International Dictionary of English v.0.48:

Pitiless \Pit"i*less\, a.
   1. Destitute of pity; hard-hearted; merciless; as, a
      pitilessmaster; pitiless elements.
      [1913 Webster]

   2. Exciting no pity; as, a pitiless condition.
      [1913 Webster] -- Pit"i*less*ly, adv. --
      Pit"i*less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pitiless
    adj 1: without mercy or pity; "an act of ruthless ferocity"; "a
           monster of remorseless cruelty" [syn: pitiless,
           remorseless, ruthless, unpitying]
    2: deficient in humane and kindly feelings [syn: pitiless,
       unkind]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "pitiless":
   atrocious, barbaric, barbarous, bestial, bloody, bowelless, brutal,
   brutish, coldhearted, cruel, cutthroat, dog-eat-dog, feral, ferine,
   ferocious, fierce, flinty, hard, hardhearted, harsh, heartless,
   inclement, inexorable, inhuman, inhumane, ironhearted, kill-crazy,
   malign, malignant, marblehearted, merciless, murderous,
   noncivilized, relentless, remorseless, ruthless, sanguinary,
   savage, stony, tameless, uncivilized, uncompassionate,
   uncompassioned, unfeeling, unforgiving, ungentle, unmerciful,
   unpitiful, unpitying, unremorseful, unsympathetic, unsympathizing,
   untamed, unyielding, vicious, wild, without mercy