demon

From Middle English demon, borrowed from Medieval Latin dēmōn/daemōn ("lar, familiar spirit, guardian spirit"), itself from Ancient Greek δαίμων (daímōn), meaning "dispenser, god, protective spirit." It pushed out the native Old English words sċucca ("evil spirit") and þyrs ("ogre or monster").

The Greek daímōn began as a neutral or even protective force, but by the time the word settled into English it had darkened into something wholly malevolent, replacing older English terms for evil beings.

It's a doublet of daimon, the more neutral-sounding English form that kept closer to the original Greek sense. In physics, 'demon' also names Maxwell's demon, a thought-experiment figure, and the particle term 'demon' derives from 'distinct electron motion' plus the suffix -on — an unrelated coinage that just happens to share the word.