What is "tansy mustard"

Tansy \Tan"sy\, n. [OE. tansaye, F. tanaise; cf. It. & Sp. tanaceto, NL. tanacetum, Pg. atanasia, athanasia, Gr. 'aqanasi`a immortality, fr. 'aqa`natos immortal; 'a priv. + qa`natos death.]

1. (Bot.) Any plant of the composite genus Tanacetum. The common tansy ( Tanacetum vulgare) has finely divided leaves, a strong aromatic odor, and a very bitter taste. It is used for medicinal and culinary purposes.

2. A dish common in the seventeenth century, made of eggs, sugar, rose water, cream, and the juice of herbs, baked with butter in a shallow dish. [Obs.]
   --Pepys.

   Double tansy (Bot.), a variety of the common tansy with the leaves more dissected than usual.

   Tansy mustard (Bot.), a plant ( Sisymbrium canescens) of the Mustard family, with tansylike leaves.

n. North American herb with bitter-tasting pinnate leaves resembling those of tansy [syn: Descurainia pinnata]