Lewis & Short

in-fīdus, a, um, adj., not to be trusted, unsafe, faithless, treacherous, false (class.): quos fidos amicos habuisset, quos infidos, Cic. Lael. 15, 53: nihil stabile quod infidum est, id. ib. 18, 65: gentes, id. Prov. Cons. 13, 33; cf.: civitas Rhodiorum, Sall. C. 51, 5: genus Numidarum, id. J. 46, 3: scurra, Hor. Ep. 1, 18, 4: societas regni, Liv. 1, 14: pax, id. 9, 45: promissa, Val. Fl. 2, 578: mare, Lucr. 2, 557; cf. Verg. G. 1, 254.
With dat.: istis infidus, Sall. H. Fragm. 1, 48, 15 Dietsch.
With ad and acc.: gens ut segnis ad pericula, ita infida ad occasiones, Tac. A. 14, 23.
Comp., sup., and adv. seem not to occur.