Idiomatic Unreasonably or stubbornly resisting change.
2
Idiomatic Fanatically opposing progress or reform.
3
Idiomatic 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.
The libertarian right of the Consservative Party, many of them diehard Europhobes, privately referred to the health minister and his advisers as "the Gestapo" for their faith in enforced lockdowns.2022, Ian McEwan, Lessons, page 644:
4
Idiomatic Strongly or fanatically determined or devoted.
I should like my Scottish correspondent to see recent statements by leading officers of certain American railways which both have been among the most extreme diehards for steam, and also have developed the most powerful and outstanding steam locomotive designs—[…].1950 January, Cecil J. Allen, “British Locomotive Practice and Performance”, in RailwAy Magazine, page 13: