knotgras

n. (Bot.) (a) a common weed with jointed stems (Polygonum aviculare); knotweed. (b) The dog grass. See under Dog. [ 1913 Webster ]

☞ 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.” [ 1913 Webster ]

We want a boy extremely for this function,
Kept under for a year with milk and knotgrass. Beau. & Fl. [ 1913 Webster ]