Mercurialis perennis
The plant's common name derives from the plant's resemblance to the unrelated "Chenopodium bonus-henricus" (Good King Henry, also known as mercury, markry, markery, Lincolnshire spinach). Since "Mercurialis perennis" is highly poisonous, it was named "dog's" mercury (in the sense of "false" or "bad"). It has also been known as boggard posy.