Definitions for knapweedˈnæpˌwid
Random House Webster's College Dictionary
any composite plant of the genus Centaurea, esp. the weedy C. nigra, having rose-purple flowers set on a dark-colored, knoblike bract.
Origin of knapweed:
1400–50; late ME knopwed=knoppe a small rounded protuberance +weed1
any of various plants of the genus Centaurea having purple thistlelike flowers
Any of various common weeds of the genus Centaurea, especially Centaurea nigra.
Origin: Alteration of knopweed.
the black centaury (Centaurea nigra); -- so called from the knoblike heads of flowers. Called also bullweed