'antiatheistic' originates from Modern English, specifically formed from the prefix 'anti-' and the adjective 'atheistic', where 'anti-' meant 'against' and 'atheistic' related to 'atheism' (lack of belief in gods).

'anti-' comes from Greek 'anti-' meaning 'against'; 'atheistic' derives from 'atheism', from Greek 'atheos' meaning 'without god', which passed into Latin and then into English. The compound 'anti-' + 'atheistic' is a modern English formation.

Initially and historically it has meant 'against atheism' or 'opposed to atheism', and this meaning has remained consistent in modern usage.