'atheistical' originates from Greek, specifically the word 'ἀ- (a-)' + 'theos', where 'a-' meant 'without' and 'theos' meant 'god'. It entered English via Latin and French forms related to 'atheist' and the adjective-forming suffixes.

'atheistical' changed from the Medieval/early modern English word 'atheist' (from Old French/Late Latin forms) and eventually became the modern English adjective 'atheistical' by adding adjective suffixes such as '-ic' and '-al'.

Initially, forms derived from 'atheos' meant 'without gods' in a literal sense; over time this evolved into the modern sense of 'denying or rejecting belief in God or gods' and, by extension, attitudes hostile to religion.