Lewis & Short

1. ūter, tris, m. (neutr. collat. form of plur. utria, Liv. And. ap. Non. p. 231, 31; gen. plur. utrium, Sall. J. 91, 1) [kindr. with uterus; v. Isid. Orig. 20, 6, 7], a bag or bottle made of an animal’s hide, a skin for wine, oil, water, etc., Plaut. Truc. 5, 11: unctos salire per utres, Verg. G. 2, 384; Curt. 7, 5, 10; Ov. Am. 3, 12, 29; Juv. 15, 20; Plin. 12, 7, 15, § 31; 28, 18, 73, § 240; Scrib. Comp. 84; Just. 1, 8, 13; Dig. 33, 6, 3.
Often inflated and used for crossing streams, Caes. B. C. 1, 48 Herz.; Liv. 21, 27, 5; Front. Strat. 3, 13, 6; Plin. 6, 29, 34, § 176; Amm. 30, 1, 9.
Poet.: crescentem tumidis infla sermonibus utrem, the swelling skin, i. e. the vain man, Hor. S. 2, 5, 98.