1. (a.) Not capable of being bent; stiff; rigid; firm; unyielding\.

2. (a.) Firm in will or purpose; not to be turned, changed, or altered; resolute; determined; unyielding; inexorable; stubborn.

3. (a.) Incapable of change; unalterable; immutable.

adamant adamantine at a standstill cast-iron certain changeless closed to constant determinate determined dogged dour dyed established fated fateful firm fixed flinty frozen fundamentalist grim hard hard and fast hard-core hard-line headstrong hidebound immobile immotile immotive immovable immutable impersuadable impersuasible impervious implacable impliable inalterable indefeasible indomitable inductile inelastic ineluctable inescapable inevasible inevitable inexorable inextensible inextensile inextensional intractable intractile intransigent invariable invincible iron ironbound ironclad ironhanded irreconcilable irremovable irresilient irresistible irrevocable loyal mulish muscle-bound necessary nonelastic nonstretchable obdurate obstinate orthodox pat pigheaded procrustean purist puristic puritan puritanic refractory relentless resistless resolute resolved rigid rigorist rigoristic rigorous rock-ribbed rockbound set settled severe single-minded sot standpat stationary staunch steadfast steady steely stern stiff stony straightlaced straitlaced strict stubborn sure sure as death sure as fate TRUE unaccommodating unadaptable unaffected unalterable unamenable unavoidable unbending unchangeable unchanging uncompliant uncompromising unconquerable uncontrollable undeflectable unextendible unextensible unflappable unflexible ungiving unimpressionable uninfluenceable unlimber unmalleable unmodifiable unmovable unmoved unmoving unpersuadable unpliable unpliant unpreventable unreceptive unrelenting unresponsive unshakable unshakeable unshaken unstoppable unsuggestible unsusceptible unswayable unswerving untractable unvaried unvarying unyielding