Same word as unadaptable:

adamant, adamantine, determined, dogged, dyed-in-the-wool, firm, fixed, hard, hard-and-fast, immovable, immutable, implacable, indomitable, inexorable, intractable, iron, obdurate, obstinate, relentless, resolute, rigid, rigorous, set, set in one's ways, single-minded, stand one's ground, staunch, steadfast, steely, stiff, strict, stringent, unadaptable, unbending, unchangeable, uncompliant, uncompromising, unrelenting, unswayable, unyielding