Other forms: demons
A demon is an evil spirit. Cast out demons, possessed by a demon, fight your demons.
In religion it is a malevolent being, and exorcism is the rite for driving one out. Figuratively, your demons are the private troubles you struggle with — fighting his demons is the standard phrase in obituaries and interviews. A demon at something means very good at it: a demon for work. Note that the Greek daimon meant a guiding spirit, neither good nor evil, and Christianity made it wicked.
1noundemonAn evil spirit.
“In the old story, a demon lived in the mountain and came down at night to steal children.”
2nounprivateA private trouble somebody struggles with.
“After the war, he struggled with his demons and couldn't sleep.”
Too few real examples of this word to estimate which sense is commonest.
demon is an evil spirit.
Greek daimon meant a divine power or guiding spirit, morally neutral. Christian Latin narrowed it to an evil one, and the word displaced the Old English scucca.
Middle English
demon
English, today
demon
Built from
From Middle English demon. From Middle English demon, a borrowing from Medieval Latin dēmōn, daemōn (“lar, familiar spirit, guardian spirit”), from Ancient Greek δαίμων (“dispenser, god, protective spirit”). Displaced native Old English sċucca (“evil spirit”) and Old English þyrs (“ogre or monster”). a doublet of daimon.
Etymology from Wiktionary, CC BY-SA