'atheisticness' originates from English, formed from the adjective 'atheistic' plus the suffix '-ness', where '-ness' meant 'state or quality'.

'atheisticness' was formed in modern English from 'atheistic' (adjective), which comes from 'atheist' + suffix '-ic'; 'atheist' ultimately comes from Greek 'atheos' via Late Latin/Old French and Middle English.

Initially, the Greek root 'atheos' meant 'without god(s)'; over time the related English terms evolved to mean 'lack of belief in gods', and 'atheisticness' came to mean the 'state or quality of lacking belief in gods'.