If you are excessively enthusiastic about something — a sports team, an actor, your religion, saving the whales, a certain brand of chocolate — then you are fanatical about it.

Fanatical comes from the word fanatic, which itself came from the Latin fanaticus, meaning "mad" or "inspired by a deity." The root word is fanum, or "temple." The original English meanings of both fanatic and fanatical implied that the person being described was insane with enthusiasm, like a religious zealot. The word fan is likely shortened from fanatic, and thus shares the same roots, but that word doesn't imply the same obsessiveness that fanatical does.

Definitions of fanatical
  1. adjective
    marked by excessive enthusiasm for and intense devotion to a cause or idea
    synonyms: fanatic, overzealous, rabid
    having or expressing strong emotions
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