From Greek 'entheos' (god-filled) — 'en-' + 'theos' (god). Originally meant divine possession; was pejorative in the 1600s.
Intense and eager enjoyment, interest, or approval.
From Latin 'enthūsiasmus,' from Greek 'enthousiasmos' (ἐνθουσιασμός, divine inspiration, ecstatic frenzy), from 'enthousiazein' (ἐνθουσιάζειν, to be inspired by a god), from 'entheos' (ἔνθεος, possessed by a god, inspired), from 'en-' (ἐν, in) + 'theos' (θεός, god). An enthusiastic person was originally 'god-filled' — possessed by divine spirit. The word was used pejoratively in the 17th-18th centuries for religious fanatics before softening to its modern positive meaning. Key roots: ἐν (en-) (Greek: "in, within"), θεός (theos) (Greek: "god").
When someone says 'I'm so enthusiastic,' they are literally claiming to have a god inside them. Greek 'entheos' (ἔνθεος) meant 'god-filled' — the state of a prophet or oracle possessed by a deity. In 17th-century England, 'enthusiasm' was a dangerous accusation: it described the uncontrolled religious fervor of Puritans, Quakers, and other 'fanatics' (from Latin 'fānum,' temple — another religious word that became negative). John Locke condemned 'enthusiasm' as