'apodictical' originates from Greek, specifically the word 'apodeiktikos' (ἀποδεικτικός), where 'apodeiknynai/apodeik-' meant 'to demonstrate or show'.

'apodeiktikos' passed into Late Latin as 'apodicticus' and then into English via scholarly Latin and French influence, producing the modern English adjective 'apodictical'.

Initially it meant 'capable of demonstration' or 'demonstrable'; over time it retained that sense but also came to be used of speech or manner that asserts absolute certainty (i.e., 'authoritatively certain').