knotgrass Syllabification: (knot·grass)Pronunciation: /ˈnätˌgras/Definition of knotgrass noun a common Eurasian plant of the dock family, with jointed creeping stems and small pink flowers. It is a serious weed in some areas. Genus Polygonum, family Polygonaceae: several species, in particular P. aviculare any of a number of other plants, especially grasses, with jointed stems. knotgrass in other Oxford dictionaries Definition of knotgrass in the British & World English dictionary