unforgiving 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. unwilling or unable to forgive or show mercy
   "a surly unforgiving old woman"
   unforgiving 
   

unforgiving in English Wiktionary