The deeper Greek adjective éntheos described someone inspired or possessed by a god. Its components lie behind later formations leading to enthusiasm.
English drew on Greek and Late Latin. The source qualifies an intervening verbal re-formation; the scored pieces are deeper than that uncertain detail.
Sources
- Merriam-Webster: enthusiasm
Word History traces the adjective and the god component, while qualifying an intervening formation.
- Merriam-Webster: en-
Prefix (2) Word History gives Greek in.