Definition of "diehard" in English

Adjective

1. Unreasonably or stubbornly resisting change.

2. Fanatically opposing progress or reform.

3. 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.

4. Strongly or fanatically determined or devoted.

Noun

1. A person with such an attitude.