diehard UK /ˈdaɪ.hɑːd/US /ˈdaɪ.hɑɹd/adjective 4noun 1forms 3 adjective4 •Complete; having no opposite opinion of anything in a particular topic of one's values; thorough of in one's beliefs.For a Roman Catholic teacher, he sure is a diehard fundamentalist. •Strongly or fanatically determined or devoted. •Unreasonably or stubbornly resisting change. •Fanatically opposing progress or reform. noun1 •A person with such an attitude. formsdiehards · more diehard · most diehard