Definition of pyrethrum
1
: any of several chrysanthemums with finely divided often aromatic leaves including ornamentals as well as important sources of insecticides
2
: an insecticide made from the dried heads of any of several Old World chrysanthemums (especially Chrysanthemum cinerariaefolium)
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Merriam-Webster unabridged
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Time Traveler for pyrethrum
The first known use of pyrethrum was circa 1543
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