Fanfrom Dictionary.com
1889, Amer.Eng., originally of baseball enthusiasts, probably a shortening of fanatic, but may be influenced by the Fancy (1807), a collective term for followers of a certain hobby or sport (especially boxing). There is an isolated use from 1682, but the modern word is likely a new formation.
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And after spending a few days with a group of guys I think Winston Churchill was on to something when he said:
"A fanatic is one who can't change his mind and won't change the subject."