Variant or alternative form of energumen; a person possessed by a spirit or demon (rare theological term).
From Greek energoumenon, a variant case form of energumenos 'one possessed.' The -on ending reflects Greek neuter noun formation, retained in some English theological vocabulary.
This word's existence as two different forms ('energumen' and 'energumenon') shows how English sometimes imports Greek words with multiple variant forms attached, creating confusing synonyms that never quite standardized.
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