'archapostate' is formed from the prefix 'arch-' (from Greek 'arkh-' meaning 'chief, principal') combined with 'apostate' (from Greek 'apostatēs').

'apostate' comes from Greek 'apostatēs' via Late Latin 'apostata' and Old French into Middle English as 'apostat'/'apostate'; the combining prefix 'arch-' comes from Greek 'arkhi-' (through Latin/Old French influence) and was attached in English to form compounds meaning 'chief ...', producing 'archapostate' in modern English usage.

Initially the elements meant 'chief' + 'one who stands away (from the faith)'; together they have retained the sense 'a principal or extreme deserter from a religion or cause', though the full compound remains rare and chiefly literary.