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):

ruthlessly
    adv 1: in a ruthless manner; "the government has been urged to
           take immediate action to deal ruthlessly with the
           strikers"