English▸ adjective stubbornly refusing to change one's opinion or course of action:
I argued this point with him, but he was obdurate.
– ORIGIN late Middle English (originally in the sense ‘hardened in sin, impenitent’): from Latin obduratus, past participle of obdurare, from ob- ‘in opposition’ + durare ‘harden’ (from durus ‘hard’).