plant used medicinally, native to Eurasia, late 14c., from rag (n.1) (see ragged), in reference to the appearance of the leaves, + wort.
Entries linking to ragwort
ragged adj.
of clothing or garments, "rough, shaggy," c. 1300 (late 12c. in surnames), past-participle adjective as though from a verb form of rag (n.1). Compare Latin pannosus "ragged, wrinkly," from pannus "piece of cloth." But ragged might reflect a broader, older meaning of the noun rag (n.1), perhaps from or reinforced by Old Norse raggaðr "shaggy," via Old English raggig "shaggy, bristly, rough" (which, Barnhart writes, "was almost surely developed from Scandinavian").
Of persons, "wearing tattered clothes," late 14c. From late 14c. of plants or leaves, "serrated." To run (someone) ragged is from 1915. Related: Raggedly; raggedness.
wort n.
"a plant," Old English wyrt "root, herb, vegetable, plant, spice," from Proto-Germanic *wurtiz (source also of Old Saxon wurt, Old Norse, Danish urt, Old High German wurz "plant, herb," German Wurz, Gothic waurts, Old Norse rot "root"), from PIE root *wrād- "branch, root." St. John's wort attested from 15c.