The Collaborative International Dictionary of English v.0.48:

Merciless \Mer"ci*less\, a.
   Destitute of mercy; cruel; unsparing; -- said of animate
   beings, and also, figuratively, of things; as, a merciless
   tyrant; merciless waves.
   [1913 Webster]

         The foe is merciless, and will not pity. --Shak.
   [1913 Webster]

   Syn: Cruel; unmerciful; remorseless; ruthless; pitiless;
        barbarous; savage.
        [1913 Webster] -- Mer"ci*less*ly, adv. --
        Mer"ci*less*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

mercilessness
    n 1: feelings of extreme heartlessness [syn: cruelty,
         mercilessness, pitilessness, ruthlessness]
    2: inhumaneness evidenced by an unwillingness to be kind or
       forgiving [syn: mercilessness, unmercifulness] [ant:
       mercifulness, mercy]