'antitheistic' originates from Modern English, specifically formed from the prefix 'anti-' (from Greek 'anti-' meaning 'against') combined with 'theistic' (from 'theism', ultimately from Greek 'theos' meaning 'god').

'antitheistic' developed as an adjective in English by combining 'anti-' with 'theistic' (derived from 'theism'); 'theism' itself was coined in the 17th-18th century from Greek 'theos' via New Latin and Modern English, and 'anti-' has been used as a productive prefix in English since Middle English.

Initially and historically it meant 'against theism' or 'opposed to belief in God(s)'; this core meaning has remained largely stable into modern usage.