Etymology
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knapweed (n.)

so called for its knobby heads, from Middle English knap "ornamental knob; bunch or tuft; a button; knot or protuberance on a tree; joint in the stalk of a plant; testicle," from Old English cnæp "top, summit of a hill," or its cognate, Old Norse knappr "a knob, button, stud."

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Definitions of knapweed

knapweed (n.)
any of various plants of the genus Centaurea having purple thistlelike flowers;
From wordnet.princeton.edu