1A tall Eurasian plant of fens and swampy meadows, Peucedanum palustre (family Apiaceae (Umbelliferae)), which has a hollow stem that exudes a milky juice when young; hog's fennel.
2More fully "Cambridge milk parsley", "†caraway-leaved milk parsley". A related plant, Selinum carvifolia, of similar habitat and appearance, but with a solid stem.
Late 18th century; earliest use found in Erasmus Darwin (1731–1802), physician and natural philosopher. From milk + parsley.