'atheistically' originates from English, specifically formed from the adjective 'atheistic' plus the adverbial suffix '-ally' (ultimately from Latin/French suffixing patterns), where the root 'atheist' comes from Greek 'atheos' meaning 'without god'.

'atheistically' changed from the adjective 'atheistic' with the addition of the adverbial ending '-ally'; 'atheistic' developed from 'atheist' (late 16th century English), which came via French 'athée' and Late Latin 'atheos' from Greek 'atheos' (a- + theos), eventually yielding modern English 'atheistic' and then 'atheistically'.

Initially, the Greek 'atheos' meant 'without god(s)'; over time the English derivatives came to mean 'relating to disbelief in God or gods', and 'atheistically' now means 'in a manner expressing atheism'.