relentless in WordNet English dictionary

1. 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"
   grim  inexorable  relentless  stern  unappeasable  unforgiving  unrelenting 
   
2. never-ceasing
   "the relentless beat of the drums"
   persistent  relentless  unrelenting