'anti-' originates from Ancient Greek, specifically the prefix 'anti-' meaning 'against', and 'religious' originates from Latin 'religiosus' (via Old French), related to 'religio' meaning 'religion' or 'reverence'.

'anti-religiously' was formed in modern English by combining the prefix 'anti-' with the adjective 'religious' and the adverbial suffix '-ly'. 'Religious' developed from Latin 'religiosus' through Old French into Middle English 'religious'.

Initially the parts signified 'against' + 'religion/reverence', and over time the combined form has come to mean 'in a manner opposed to religion'—a meaning that has remained stable in modern usage.