'antidemonic' originates from Greek combining form 'anti-' (from Greek 'anti' meaning 'against') and the adjective 'demonic' (from Late Latin 'demonicus' and Greek 'daimonios'), where 'anti-' meant 'against' and 'daimon'/'daimonion' meant 'spirit' or 'divine being'.

'antidemonic' was formed in modern English by adding the prefix 'anti-' to the existing adjective 'demonic'. The element 'demonic' comes into English via Middle English from Old French/Late Latin ('demonicus'), ultimately from Greek 'daimonios' (related to 'daimon'). Over time the components combined in modern English to produce 'antidemonic'.

Initially, the Greek 'daimon' referred broadly to a 'spirit' or 'divine being' (not necessarily evil); over time 'demon' came to mean an evil supernatural being, and 'antidemonic' evolved to mean specifically 'against evil/demonic forces' or, figuratively, 'opposed to destructive or malevolent influences'.