define milk thistle
The noun MILK THISTLE has 2 senses: 1. 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. 2. tall Old World biennial thistle with large clasping white-blotched leaves and purple flower heads; naturalized in California and South America. Familiarity information: MILK THISTLE used as a noun is rare.