'arch-eunuch' originates from Greek elements: the prefix 'archi-' (meaning 'chief') and the word 'eunoukhos' (Ancient Greek εὐνοῦχος), where 'eune' meant 'bed' and 'oukhos' meant 'keeper' (via Latin 'eunuchus').

'arch-eunuch' changed as the component 'eunoukhos' passed into Latin as 'eunuchus', then into Old French/Medieval and Middle English as 'eunuch', and later English speakers combined it with the Greek-derived prefix 'arch-' to form the compound 'arch-eunuch'.

Initially it referred to a 'bed-keeper' (a eunuch charged with guarding the bedchamber); over time it evolved to mean the 'chief' or highest-ranking eunuch in a court.