FOOL'S PARSLEY, in botany, the popular name for Aethusa Cynapium, of the family Umbelliferae, a common weed in cultivated ground in Great Britain and sparingly naturalized in the eastern United States and Canada. It is an annual herb, with a f usif orm root and a smooth hollow branched stem i to 2 ft. high, with much divided (ternately pinnate) smooth leaves and small compound umbels of small irregular white flowers. The plant has a nauseous smell, and, like various other members of the family (e.g., hemlock), is poisonous.