fanatic

Words with a similar base: fanatic, fanatical

'fanatic'

[fanatic.n.01] as in: a person motivated by irrational enthusiasm (as for a cause); --Winston Churchill

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Examples:
A fanatic is one who can't change his mind and won't change the subject

'fanatic'

[fanatic.s.01] as in: marked by excessive enthusiasm for and intense devotion to a cause or idea

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Examples:
rabid isolationist