monk's rhubarb
/ˌmʌŋks ˈruːbɑːb/(also monks' rhubarb, monk's-rhubarb)
noun
Either of two species of dock formerly grown as medicinal plants or pot-herbs, (originally) patience dock, Rumex patientia, and (in later use) R. pseudoalpinus.
Origin
Late 16th century; earliest use found in William Langham (fl. 1597), doctor and herbalist. From the genitive of monk + rhubarb, after post-classical Latin Rhabarbarum Monachorum.