Source : Webster's Revised Unabridged Dictionary (1913)

Unmerciful \Un*mer"ci*ful\, a.
   Not merciful; indisposed to mercy or grace; cruel; inhuman;
   merciless; unkind. -- {Un*mer"ci*ful*ly}, adv. --
   {Un*mer"ci*ful*ness}, n.

Source : WordNet®

unmerciful
     adj : having or showing no mercy; "the merciless enemy"; "a
           merciless critic"; "gave him a merciless beating" [syn:
            {merciless}] [ant: {merciful}]