obdurate

Concise Oxford English Dictionary © 2008 Oxford University Press:
obdurate /ˈɒbdjʊrət/

▶adjective stubbornly refusing to change one's opinion or course of action.

– derivatives
obduracy noun,
obdurately adverb,
obdurateness noun.

– origin ME (orig. in the sense ‘impenitent’): from L. obduratus, past part. of obdurare, from ob- ‘in opposition’ + durare ‘harden’.