Lewis & Short

mūnĭa (archaic form moenia: moenia praeter aedificia significant etiam et munia, hoc est officia, Paul. ex Fest. p. 151 Müll.), ōrum (class. only in the form munia, nom. and acc.), n. [Sanscr. root , bind; that to which one is bound; cf. Gr. ἀμύνω; also moenia, murus, munus, immunis, etc.], duties, functions, esp. official or professional duties (class.; syn.: officia, munera): munia candidatorum, Cic. Mur. 35, 73: qui suis cervicibus tanta munia atque rem publicam sustinent, id. Sest. 66, 138: cuncti omnium ordinum extrema munia sequi, Sall. Fragm. ap. Non. 137, 12: belli pacisque munia facere, Liv. 1, 42: senatūs, magistratuum, legum in se trahere, Tac. A. 1, 2; cf.: munia consulatūs obire, id. ib. 2, 26: munia ducis implere, id. H. 1, 62: vitae servare munia, Hor. Ep. 2, 2, 131.
Post-class. forms, gen.: OB HAEC INSIGNIA GENERA MVNIORVM, Inscr. Spon. Miscell. Antiq. p. 36 (A. D. 394), but gen. muniūm, Tert. Cor. Mil. 11.
Dat. munibus, Claud. Laud. Stil. 3, 76: muniis, Amm. 31, 2, 20; Serv. Verg. A. 12, 559; Cod. Just. 8, 10, 8 al.