relentless

Adjective

Synonyms: grim inexorable stern unappeasable unforgiving unrelenting persistent

WordNet 3.0

Adjective (2)

not to be placated or appeased or moved by entreaty“grim determination”“grim necessity”“Russia's final hour, it seemed, approached with inexorable certainty”“relentless persecution”“the stern demands of parenthood”Synonyms: grim inexorable stern unappeasable unforgiving unrelenting
never-ceasing“the relentless beat of the drums”Synonyms: persistent unrelenting

Webster's Dictionary (GCIDE)

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]