Synonyms and related words :
adamant, adamantine, immovable, immutable, inelastic, inextensible, inextensile, inextensional, inflexible, intractable, intractile, irresilient, nonelastic, nonstretchable, stubborn, unalterable, unbending, unextendible, unextensible, unflexible, ungiving, unlimber, unmalleable, unpliable, unpliant, untractable, unyielding