[mus.tard] n [ME, fr. OF mostarde, fr. moust must, fr. L mustum] (13c) 1 a: a pungent yellow powder of the seeds of any of several common mustards (Brassica hirta, B. nigra, or B. juncea) used as a condiment or in medicine as a stimulant and diuretic, an emetic, or a counterirritant b slang: zest
2: any of several herbs (genus Brassica of the family Cruciferae, the mustard family) with lobed leaves, yellow flowers, and linear beaked pods
3: a dark to moderate yellow -- mus.tardy adj