Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sīnum, i, n. (masc. collat. form sīnus, Plaut. Curc. 1, 1, 82; id. Rud. 5, 2, 31; for the form of the abl., sinu, Varr. ap. Non. 547, 23, we should perh. substitute sino; cf. Prisc. p. 714 P.) [1. sinus], a large, round drinking-vessel with swelling sides (like our bowls), esp. for wine (cf. Cic. Fragm. ap. Schol. Veron. ad Verg. E. 7, 33), Varr. L. L. 5, § 123 Müll.; id. ap. Non. 547, 23; Plaut. Curc. 1, 1, 75; 1, 1, 82; id. Rud. 5, 2, 32; Atta ap. Serv. Verg. E. 7, 33; Valgius ap. Philarg. Verg. G. 3, 177; Col. 7, 8, 2 al.; cf. Becker, Gallus, 3, p. 225 (2d edit.).