1) Adamant 2) Grim 3) Inexorable 4) Merciless 5) Relentless 6) Unappeasable 7) Unforgiving 8) Unmerciful 9) Unmitigable 10) Unplacable 11) Unrelenting 1) French word used in English 2) Grim 3) Impossible to appease 4) Incapable of being placated 5) Inexorable 6) Merciless 7) Relentless 8) Stern 9) Unappeasable 10) Unforgiving 11) Unmerciful 12) Unmitigable 13) Unrelenting 
• (a.) Not placable; not to be appeased; incapable of being pacified; inexorable; as, an implacable prince. • (a.) Incapable of ebign relieved or assuaged; inextinguishable.
Im·pla'ca·ble adjective [ Latin implacabilis ; prefix im- not + placabilis : confer French implacable . See Placable .] 1. Not placable; not to be appeased; incapable of being pacified; inexorable; as, an implacable prince. « I see thou art impla...
incapable of being placated unpleasable 
HMS Implacable was a British first-class battleship of 15000 tons launched in 1899. The name was first given to the French ship Duguay Trouin captured in 1805 and still used a hundred years later as a training ship at Devonport. *Implacable II
[adj] - impossible to placate
adjective incapable of being placated; `an implacable enemy`
not to be appeased, mollified, or pacified; inexorable: an implacable enemy.
incapable of being appeased or pacified incapable of being appeased or pacified[Intelligent words] incapable of being appeased or pacified