1 adamant, brassbound, dyed-in-the-wool, firm, fixed, hard and fast, immovable, immutable, implacable, inexorable, intractable, iron, obdurate, obstinate, relentless, resolute, rigorous, set, set in one's ways, steadfast, steely, stiff-necked, strict, stringent, stubborn, unadaptable, unbending, unchangeable, uncompromising, unyielding
2 hard, hardened, inelastic, nonflexible, rigid, stiff, taut
Antonyms
elastic, flexible, irresolute, lissom(e), movable, pliable, pliant, supple, variable, yielding