'antireligious' originates from a Modern English compound, combining Greek 'anti' (ἀντί) meaning 'against' and English 'religious', which ultimately comes from Latin 'religio' meaning 'reverence' or 'religious obligation'.

'religious' came into English via Latin 'religiosus' (from 'religio') through Old/Middle English forms; in Modern English the prefix 'anti-' was attached to 'religious' to form 'antireligious'.

Initially it meant 'against religion' and over time has retained that core sense of opposition or hostility toward religion.