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From The Collaborative International Dictionary of English v.0.48:
Knotgrass \Knot"grass`\, n. (Bot.) (a) a common weed with jointed stems (Polygonum aviculare); knotweed. (b) The dog grass. See under Dog. [1913 Webster] 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." [1913 Webster] We want a boy extremely for this function, Kept under for a year with milk and knotgrass. --Beau. & Fl. [1913 Webster]