mermaid weed
US
noun
Any of several small creeping North American or Caribbean swamp plants of the genus Proserpinaca (family Haloragaceae), especially P. palustris, having dimorphic leaves with the submerged ones pinnatifid, and P. pectinata, having all the leaves pinnatifid.
Origin
Early 19th century; earliest use found in Amos Eaton (1776–1842).