Any of various plants whose leaves or extracts were traditionally used as remedies for treating fevers.
From 'fever' plus 'weed,' a general folk taxonomy term. In older English, 'weed' simply meant any plant or herb, not necessarily an unwanted one—it appears in many herbal remedy names like 'mugwort' and 'ragweed'.
The word 'weed' in 'feverweed' has nothing to do with weeds being unwanted—it comes from Old English 'weod' meaning herb or plant! Medieval herbalists used '-weed' for all their medicinal plants, showing how meaning shifts over centuries as the same root gets reused.
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