1. (a.) Not to be appeased; incapable of being pacified; inexorable; as, an implacable prince\.

2. (a.) Incapable of being relieved or assuaged; inextinguishable.

adamant adamantine avenging cast-iron cruel dour firm flinty grim grudgeful hard hard-core immovable immutable impliable inelastic inexorable inflexible intractable intransigent iron irreconcilable merciless mortal pitiless punitive punitory rancorous relentless retaliatory revanchist revengeful rigid rigorous rock-ribbed ruthless steely stern stiff unaffected unalterable unappeasable unbending unchangeable uncompassionate uncompromising unflinching unforgiving ungiving unmoved unrelenting unsympathetic unyielding vengeful vindicatory vindictive