Lewis & Short

Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.

Allĭa (more correct than Alĭa; cf. Wagner, Orthogr. Vergil. p. 415 sq.), ae, f., a little river eleven miles northwards from Rome, near Crustumerium, in the country of the Sabines, passing through a wide plain (cf. Mann. Ital. 1, 520; Müll. Roms Camp. 1, 138; 141 sq.); it was made memorable by the terrible defeat of the Romans by the Gauls A. U. C. 365, XV. Kal. Sextil. (18 July).
Hence, Alliensis, e, adj., of or pertaining to Allia: dies, of this battle, considered ever after as a dies nefastus, Liv. 5, 37-39; 6, 1; Cic. Att. 9, 5; Verg. A. 7, 717; Luc. 7, 408; Suet. Vit. 11; cf. Paul. ex Fest. p. 7 Müll.