Synonyms and related words :
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