unyielding

Synonyms: adamant dead set on determined firm fixed hard hard-core hardheaded hard-line hardnosed headstrong immalleable immovable implacable impliable inexorable inflexible intractable locked in merciless mulish obdurate obstinate pertinacious pigheaded refractory relentless rigid ruthless singleminded solid staunch stiff stiff-necked stubborn tough unbending uncompliant uncompromising unmovable unrelenting unswayable unwavering

Antonyms: flexible irresolute surrendering yielding