'post-office' is a compound of English 'post' + 'office'. 'post' originates from Middle French 'poste' (from Italian 'posta'), ultimately from Latin 'positus' (past participle of 'ponere'), where the root 'pon-' meant 'to place/put'. 'office' originates from Latin 'officium', meaning 'service' or 'duty'.

'post' entered English via Middle French/Italian in the late medieval period to mean a station or relay for couriers; as organized mail services developed in the 16th–18th centuries, the compound 'post office' came to denote the place where postal business was conducted.

Originally it could refer to a relay station or place where couriers changed horses or handed on messages; over time it evolved into the modern sense of a public building and the organization that handles mail.