Lewis & Short

sĭnāpi (sĭnāpe, Apic. 6, 4; 8, 7), is, n. (collat. form sĭnāpis, is, f., Plaut. Ps. 3, 2, 28; Col. poët. 10, 122; acc. sinapim, Pall. Oct. 11, 2), = σίναπι, mustard, Col. 11, 3, 29; Plin. 19, 8, 54, § 170; 20, 22, 87, § 236; Enn. ap. Macr. S. 6, 5; Cels. 2, 21; 2, 22; 2, 31; Petr. 66, 7.
Gen. sinapis, Col. 11, 3, 29; Plin. 21, 12, 41, § 71.
Abl. sinapi, Plaut. Truc. 2, 2, 60: sinape, Varr. R. R. 1, 59, 4.