Synonyms and related words :
adamant, adamantine, cast-iron, dour, firm, flinty, fundamentalist, grim, hard, hard-core, hard-line, hidebound, immovable, immutable, implacable, impliable, inelastic, inexorable, inflexible, intransigent, iron, ironbound, ironclad, ironhanded, irreconcilable, muscle-bound, obdurate, obstinate, orthodox, procrustean, purist, puristic, puritan, puritanic, relentless, rigid, rigorist, rigoristic, rigorous, rock-ribbed, rockbound, single-minded, steely, stern, stiff, straightlaced, straitlaced, stubborn, unaffected, unalterable, unbending, unchangeable, uncompliant, ungiving, unmoved, unrelenting, unyielding