Lewis & Short

Insŭber, bris, adj. (Insubris for Insuber, Spart. Julian. 1; gen. plur. Insubrium, Plin. 10, 29, 41, § 77: Insubrum, id. 3, 17, 21, § 125), of or belonging to Insubria, a country in the neighborhood of Milan, Insubrian: eques, Liv. 22, 6, 3.
Subst.: Insŭber, an Insubrian: Insuber quidam fuit, Cic. Pis. 15, 34.
Plur.: Insŭbres, ium, m., the Insubrians, Cic. Balb. 14, 32; Liv. 30, 18, 1; 5, 34, 9; Plin. 3, 17, 21, § 124 sq.