officium

Officium, officii, n., is second declension, worn down from opificium, "the doing of work". Its range is wider than "duty": a moral obligation, a service or kindness done for somebody, an official post, and in the plural the courtesies you owe - attending a friend at court, seeing him off on a journey. In officio esse or manere is to stay loyal, which is why officium turns up in every account of a wobbling province.

Two notes. Cicero's De Officiis is about obligations, not office life; the word is not a room until modern Latin. And in the Church, officium divinum is the daily round of prayer, the Divine Office, from which the Book of Hours and English "officiate" both descend.