Source : Webster's Revised Unabridged Dictionary (1913)

Knotgrass \Knot"grass`\, n. (Bot.)
   (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.

Source : WordNet®

knotgrass
     n : low-growing weedy grass with spikelets along the leaf stems
         [syn: {Paspalum distichum}]