INFLEXIBLE, OBDURATE, ADAMANT mean unwilling to alter a predetermined course or purpose.

INFLEXIBLE implies rigid adherence or even slavish conformity to principle.

e.g. inflexible in their demands

OBDURATE stresses hardness of heart and insensitivity to appeals for mercy or the influence of divine grace.

e.g. obdurate in his refusal to grant clemency

ADAMANT implies utter immovability in the face of all temptation or entreaty.

e.g. adamant that the work should continue

What is the difference between "obdurate" and "obstinate" ? "obdurate" vs "obstinate" ? | HiNative

stubbornly refusing to change one's opinion or course of action:

｟かたく｠ 頑固［頑迷］な, 硬骨な(stubborn)

e.g. I argued this point with him, but he was obdurate.

obduracy |ˈäbd(y)ərəsē| noun

obdurately |ˈäbd(y)ərətlē| adverb

obdurateness noun

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’).