The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tansy mustard
    n 1: North American herb with bitter-tasting pinnate leaves
         resembling those of tansy [syn: tansy mustard,
         Descurainia pinnata]