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]