Definition of mermaid weed in US English:

mermaid weed

noun

US
  • 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).