Synonyms and related words :
adamant, adamantine, cast-iron, dour, firm, flinty, grim, hard, hard-core, immovable, immutable, implacable, impliable, inductile, inexorable, inextensible, inextensile, inextensional, inflexible, intractable, intractile, intransigent, iron, irreconcilable, irresilient, nonelastic, nonstretchable, relentless, rigid, rigorous, rock-ribbed, steely, stern, stiff, stubborn, unaffected, unalterable, unbending, unchangeable, uncompromising, unextendible, unextensible, unflexible, ungiving, unlimber, unmalleable, unmoved, unpliable, unpliant, unrelenting, untractable, unyielding