'antitheistic' originates from Modern English, formed from the Greek prefix 'anti-' and the adjective 'theistic', where 'anti-' meant 'against' and the Greek 'theos' meant 'god'.

'antitheistic' developed in the 19th century as a formation from the noun 'antitheism' and the noun/adjective elements 'antitheist'/'theistic', eventually becoming the modern adjective from which 'antitheistically' is derived.

Initially it meant 'against theistic doctrines or theism', and over time it has retained that basic sense of opposition to belief in God or gods.