Lewis & Short

ancīle (also ancŭle after ἀγκύλιον in Plut. Num.), is, n. (gen. plur. ancilium, Tac. H. 1, 89; but anciliorum, Hor. C. 3, 5, 10; cf. Consent. p. 1898 P.) [prob. from ἀγκύλος, crooked, curved; v. ango], a small oval shield, Verg. A. 7, 188 Serv.; Luc. 9, 480; but specif. the shield that was said to have fallen from heaven in the reign of Numa (hence, caelestia arma, Liv. 1, 20), and on the preservation of which the prosperity of Rome was declared to depend; whereupon Numa caused eleven others exactly like it to be made by the artist Mamurius Veturius. so that if the genuine one was lost, the fact could not be known. These shields were carefully preserved by the Salian priests in the temple of Mars, and every year in March carried about in solemn procession (ancilia movere), and then returned to their place (ancilia condere), Ov. F. 3, 377; Liv. 1, 20; Verg. A. 8, 664; Tac. H. 1, 89; Suet. Oth. 8; Inscr. Orell. 2244; v. Smith, Dict. Antiq.
Note: Adj.: clipeis ancilibus, Juv. 2, 126: arma ancilia, Val. Max. 1, 1, 9.