Synonyms and related words :
adamant, adamantine, bowelless, cast-iron, certain, cruel, dog-eat-dog, dogged, dour, fated, fateful, firm, flinty, fundamentalist, grim, hard, hard-core, harsh, heartless, hidebound, immobile, immovable, immutable, implacable, impliable, inclement, indefeasible, inelastic, ineluctable, inescapable, inevasible, inevitable, inflexible, intransigent, iron, ironbound, ironclad, ironhanded, irreconcilable, irresistible, irrevocable, merciless, muscle-bound, necessary, obdurate, obstinate, orthodox, pitiless, procrustean, purist, puristic, puritan, puritanic, relentless, remorseless, resistless, resolute, rigid, rigorist, rigoristic, rigorous, rock-ribbed, rockbound, ruthless, single-minded, steely, stern, stiff, straightlaced, straitlaced, stubborn, sure, sure as death, sure as fate, unaffected, unalterable, unavoidable, unbending, unchangeable, uncompassionate, uncompassioned, uncompromising, uncontrollable, undeflectable, unfeeling, unforgiving, ungiving, unmerciful, unmoved, unpitiful, unpitying, unpreventable, unrelenting, unremorseful, unstoppable, unsympathetic, unsympathizing, unyielding, without mercy