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):

pitilessness
    n 1: feelings of extreme heartlessness [syn: cruelty,
         mercilessness, pitilessness, ruthlessness]
    2: mercilessness characterized by a lack of pity [syn:
       pitilessness, ruthlessness]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "pitilessness":
   acuteness, animality, atrocity, barbarity, bloodlust, brutality,
   callosity, callousness, coldbloodedness, coldheartedness, coldness,
   destructiveness, extremity, ferociousness, fierceness, force,
   furiousness, hardheartedness, hardness, hardness of heart,
   harshness, heart of stone, heartlessness, impetuosity, inclemency,
   inhumanity, insensitivity, intensity, malignity, mercilessness,
   mindlessness, murderousness, obduracy, rigor, roughness, savagery,
   severity, sharpness, terrorism, unfeeling, ungentleness,
   unmercifulness, unnaturalness, unresponsiveness, vandalism,
   vehemence, venom, viciousness, violence, virulence