The Collaborative International Dictionary of English v.0.48:

Remorseless \Re*morse"less\, a.
   Being without remorse; having no pity; hence, destitute of
   sensibility; cruel; insensible to distress; merciless.
   "Remorseless adversaries." --South. "With remorseless
   cruelty." --Milton.
   [1913 Webster]

   Syn: Unpitying; pitiless; relentless; unrelenting;
        implacable; merciless; unmerciful; savage; cruel.
        [1913 Webster] -- Re*morse"less*ly, adv. --
        Re*morse"less*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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