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]
"devotee," 1889, American English, originally of baseball enthusiasts, probably a shortening of fanatic, but it may be influenced by the fancy, a collective term for followers of a certain hobby or sport (especially boxing); see fancy (n.). There is an isolated use from 1682, but the modern word likely is a late 19c. formation. Fan mail attested from 1920, in a Hollywood context; Fan club attested by 1930.
Before the close of the republic, an enthusiastic partisan of one of the factions in the chariot races flung himself upon the pile on which the body of a favourite coachman was consumed, and perished in the flames. [W.E.H. Lecky, "A History of European Morals," 1869]
It forms all or part of: apotheosis; atheism; atheous; Dorothy; enthusiasm; fair (n.) "a stated market in a town or city;" fanatic; ferial; feast; fedora; -fest; festal; festival; festive; festoon; Festus; fete; fiesta; henotheism; monotheism; pantheism; pantheon; polytheism; profane; profanity; Thea; -theism; theist; theo-; theocracy; theodicy; Theodore; Theodosia; theogony; theology; theophany; Theophilus; theosophy; theurgy; tiffany; Timothy.
It is the hypothetical source of/evidence for its existence is provided by: Greek theos "god;" Latin feriae "holidays," festus "festive," fanum "temple."