Merlin's grass
/ˈməːlɪnz ɡrɑːs/ /ˈməːlɪnz ɡras/noun
Any of the quillworts (genus Isoetes), plants allied to ferns which characteristically grow on lake bottoms; especially the European species I. lacustris and I. echinospora.
Origin
Early 19th century; earliest use found in James Edward Smith (1759–1828), botanist. Probably from the genitive of the name of Merlin + grass, apparently a mistranslation of Welsh gwair merllyn quillwort from gwair grass, hay + merllyn standing water, lake, pond; from either merf still, stagnant or marw dead + llyn lake, pool).