What does "knotgrass" mean?

Knotgrass: (a) a common weed with jointed stems (Polygonum aviculare); knotweed. (b) The dog grass. See under Dog. Note: An infusion of Polygonum aviculare was once supposed to have the effect of stopping the growth of an animal, and hence it was called, as by Shakespeare, "hindering knotgrass." We want a boy extremely for this function, Kept under for a year with milk and knotgrass. Beau. & Fl.