Source : Webster's Revised Unabridged Dictionary (1913)

Knapweed \Knap"weed`\, n. (Bot.)
   The black centaury ({Centaurea nigra}); -- so called from the
   knoblike heads of flowers. Called also {bullweed}.

Source : WordNet®

knapweed
     n : any of various plants of the genus Centaurea having purple
         thistlelike flowers