milk thistle

(noun) any of several Old World coarse prickly-leaved shrubs and subshrubs having milky juice and yellow flowers; widely naturalized; often noxious weeds in cultivated soil. Synonyms: sow thistle, milk thistle.

(noun) tall Old World biennial thistle with large clasping white-blotched leaves and purple flower heads; naturalized in California and South America. Synonyms: milk thistle, lady's thistle, Our Lady's mild thistle, holy thistle, blessed thistle, Silybum marianum.