The Collaborative International Dictionary of English v.0.48:

Relentless \Re*lent"less\, a.
   Unmoved by appeals for sympathy or forgiveness; insensible to
   the distresses of others; destitute of tenderness;
   unrelenting; unyielding; unpitying; as, a prey to relentless
   despotism.
   [1913 Webster]

         For this the avenging power employs his darts, . . .
         Thus will persist, relentless in his ire. --Dryden.
   [1913 Webster] -- Re*lent"less*ly, adv. --
   Re*lent"less*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

relentlessly
    adv 1: in a relentless manner; "he worked relentlessly" [syn:
           relentlessly, unrelentingly]