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