Hypertext Webster Gateway: "knapweed"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
Knapweed \Knap"weed`\, n. (Bot.)
The black centaury ({Centaurea nigra}); -- so called from the
knoblike heads of flowers. Called also {bullweed}.
From WordNet (r) 1.7 (wn)
knapweed
n : any of various plants of the genus Centaurea having purple
thistlelike flowers
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