'archpirate' is a compound formed in Modern English from the prefix 'arch-' (from Greek/Latin via Old French meaning 'principal' or 'chief') and 'pirate' (from Latin 'pirata' and Greek 'peiratēs').

'pirate' comes from Greek 'peiratēs' (πειρατής) meaning 'one who attacks' or 'brigand', which became Latin 'pirata', passed into Old French and Middle English as 'pirat'/'pirate'; the productive English prefix 'arch-' (from Greek 'arkhos' via Latin) combined with 'pirate' in Modern English to form the compound 'archpirate'.

Originally the elements referred to 'chief' and 'one who attacks at sea'; over time the compound came to mean specifically a leading or supreme pirate (or, figuratively, the chief perpetrator of piracy).