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Webster 1913 Edition

Noun.

(Bot.)

The black centaury (

Centaurea nigra

); – so called from the knoblike heads of flowers. Called also

bullweed

Noun.

nap'weed. A plant of the genus Centaurea, so called probably from knap, a button.

knapweed ‎(plural knapweeds)

Any of various common weeds of the genus Centaurea
1. especially Centaurea nigra.

Centaurea