adjective: obdurate
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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 Latinobduratus, past participle of obdurare, from ob- ‘in opposition’ + durare ‘harden’ (fromdurus ‘hard’).
How my heart sings 