'atheisticalness' originates from English, specifically the word 'atheistical' combined with the suffix '-ness', where the prefix 'a-' (from Greek) meant 'not' and the root 'theos' meant 'god'.

'atheisticalness' formed from the adjective 'atheistical' (a variant of 'atheistic') plus the noun-forming suffix '-ness'. The adjective 'atheistic' ultimately derives from Greek 'atheos' ('a-' + 'theos') → Late Latin/Latinized form 'atheismus' → Middle English 'atheism' → modern English 'atheistic'/'atheistical', and then the derived noun 'atheisticalness' appeared in modern English.

Initially related to the literal sense 'without gods' (from Greek 'atheos'), the terms moved to express a personal or doctrinal lack of belief in deities; the noun-form now denotes the quality or state of that unbelief.