Demonic means belonging to or resembling a devil. Demonic possession, a demonic laugh, demonic energy.
It is used seriously in theology and folklore and loosely everywhere else, where demonic energy is a compliment about somebody's work rate. Demonic possession is a belief with a long history and a continuing practice of exorcism in several churches. Note demon; demonise, meaning to portray somebody as evil, which is a common political criticism; and that the Greek daimon originally meant a spirit of any kind, good or bad.
1adjectivedemonicBelonging to or resembling a devil.
“The old painting shows demonic figures dragging sinners into the flames.”
2adjectivewildlyWildly intense.
“She cleaned the house with demonic energy before her parents arrived.”
demonic is a devilish quality.
Greek daimonikos comes from daimon, a spirit or divine power — not necessarily evil. Socrates spoke of his daimon as a guiding voice. Christian writing narrowed the word to the devil's side, and English inherited only that half.
Ancient Greek
δαιμονικός — possessed by a demon, sent by a demon
Latin
daemonicus
English, today
demonic
From Latin daemonicus, from Ancient Greek δαιμονικός (“possessed by a demon, sent by a demon”), from δαίμων, equivalent to demon + ic. a doublet of daimonic.
Etymology from Wiktionary, CC BY-SA