fanatic (n.)

1520s, "insane person," from Latin fanaticus "mad, enthusiastic, inspired by a god," also "furious, mad," originally, "pertaining to a temple," from fanum "temple, shrine, consecrated place," related to festus "festive" (see feast (n.)). Meaning "zealous person, person affected by enthusiasm" is from 1640s. 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]

updated on September 25, 2018

Definitions of fanatic from WordNet
fanatic (n.)
a person motivated by irrational enthusiasm (as for a cause); "A fanatic is one who can't change his mind and won't change the subject"--Winston Churchill;
Synonyms: fiend
fanatic (adj.)
marked by excessive enthusiasm for and intense devotion to a cause or idea;
Synonyms: fanatical / overzealous / rabid
Etymologies are not definitions. From, not affiliated with etymonline.