The Collaborative International Dictionary of English v.0.48:

Ruthless \Ruth"less\, a.
   Having no ruth; cruel; pitiless.
   [1913 Webster]

         Their rage the hostile bands restrain,
         All but the ruthless monarch of the main. --Pope.
   [1913 Webster] -- Ruth"less*ly, adv. -- Ruth"less*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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