'archapostle' is formed from the Greek prefix 'archi-' (chief) combined with 'apostle' from Greek 'apostolos' (one sent off), ultimately borrowed into English via Late Latin and Old English usage of 'apostle'.

'apostolos' (Greek) → 'apostolus' (Latin) → 'apostle' (Old/Middle English); the compound with 'archi-' (Greek 'arkhi-') was formed in English to mean 'chief apostle'.

Originally it denoted a literal 'chief apostle' (often applied to prominent early Christian figures); over time it has also been used figuratively to mean the leading advocate or founder of a movement.