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1520s, "insane person," from Latin fanaticus "mad, enthusiastic, inspired by a god," also "furious, mad," originally, "pertaining to a temple," from fanum "temple," related to festus "festive" (see feast). Meaning "zealous person" is mid-17c. As an adjective, in English, 1530s, "furious;" meaning "characterized by excessive enthusiasm," especially in religion (of Nonconformists), is from 1640s.
A fanatic is someone who can't change his mind and won't change the subject. [attributed to Winston Churchill]