It is important to distinguish perennial sow thistle from annual sow thistle and prickly lettuce.
British Dictionary definitions for sow thistle
any of various plants of the Old World genus Sonchus, esp S. oleraceus, having milky juice, prickly leaves, and heads of yellow flowers: family Asteraceae (composites) Also called milk thistle, (NZ) puha, (NZ) rauriki
C13: from sugethistel, perhaps variant of Old English thugethistel, thuthistel thowthistle, a dialect name of the sow thistle. See sow², thistle