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):

pitilessly
    adv 1: without pity; in a merciless manner; "he was mercilessly
           trounced by his opponent in the House" [syn:
           mercilessly, pitilessly, unmercifully,
           remorselessly]