'apostatical' originates from Latin, specifically the word 'apostaticus', from Greek 'apostatikos' (ἀποστατικός), where 'apo-' meant 'away from' and 'stasis'/'statos' related to 'standing' or 'a standing apart'.

'apostatical' came into English via Late Latin and Middle French/Medieval Latin forms; the Greek 'apostatikos' passed into Late Latin as 'apostaticus' and then into English as the adjective 'apostatical'.

Initially it meant 'pertaining to rebellion or defection (standing away)', and over time it narrowed to mean specifically 'relating to apostasy — abandoning belief or faith'.