noun
a person who strongly opposes change or refuses to give up on something
adjective
stubbornly resistant to change or unwavering in belief or opinion
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The diehard fans of the team have been supporting them through thick and thin.
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My uncle is a diehard fan of that band and has seen them in concert ten times.
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She's a diehard foodie and will travel across town just to try a new restaurant.
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He's a diehard optimist, always finding the silver lining in every situation.