What is "treacle mustard"

Treacle \Trea"cle\ (tr[=e]"k'l), n. [OE. triacle a sovereign remedy, theriac, OF. triacle, F. th['e]riaque (cf. Pr. triacla, tiriaca, Sp. & It. triaca, teriaca), L. theriaca an antidote against the bite of poisonous animals, Gr. ?, fr. ? of wild or venomous beasts, fr. qhri`on a beast, a wild beast, dim. of qh`r a beast. Cf. Theriac.]

1. (Old Med.) A remedy against poison. See Theriac, 1.

   We kill the viper, and make treacle of him.
   --Jer. Taylor.

2. A sovereign remedy; a cure. [Obs.]

   Christ which is to every harm treacle.
   --Chaucer.

3. Molasses; sometimes, specifically, the molasses which drains from the sugar-refining molds, and which is also called sugarhouse molasses.

   Note: In the United States molasses is the common name; in England, treacle.

4. A saccharine fluid, consisting of the inspissated juices or decoctions of certain vegetables, as the sap of the birch, sycamore, and the like.

   Treacle mustard (Bot.), a name given to several species of the cruciferous genus Erysimum, especially the Erysimum cheiranthoides, which was formerly used as an ingredient in Venice treacle, or theriac.

   Treacle water, a compound cordial prepared in different ways from a variety of ingredients, as hartshorn, roots of various plants, flowers, juices of plants, wines, etc., distilled or digested with Venice treacle. It was formerly regarded as a medicine of great virtue.
   --Nares.

   Venice treacle. (Old Med.) Same as Theriac, 1.

n. (context botany English) Any of several species of the cruciferous genus ''Erysimum'', especially ''Erysimum cheiranthoides'', which was formerly used as an ingredient in Venice treacle, or theriac.