The word enthusiasm was first coined in 1603 from the Middle French word enthousiasme, and that came from the Latin term enthusiasmus, all with the same meaning. However, definition begins to change as the word is traced back to Ancient Greek enthousiasmos, which literally meant "possessed by gods". In the context of this phrase, somebody could be so motivated or energetic about something that it's like they were possessed by the gods themselves. The word grew a little weaker in meaning over time, but that's the origin. Since en- is a prefix meaning "in", the root thous has to do with theos, "god", and the rest is a useless suffix, by the definition of enthousiasmos there is literally a "god in" you. En is from a Proto-Indo-European reconstruction with the same intonations and denotations, and theos comes from PIE dhes, which generally had to do with religious words. Usage of the word enthusiasm has been declining since an 1899 peak in literature; maybe godly possessions are declining.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.