adder's grass
/ˈadəz ˌɡrɑːs/ /ˈadəz ˌɡras/noun
Any of several plants thought to resemble a snake in some way, or used to treat snakebites; especially adder's tongue, genus Ophioglossum, and †bistort, Persicaria bistorta ( obsolete ); compare adderwort. Now historical and rare.
Origin
Late Middle English; earliest use found in Medical Recipes. In some forms from adder + grass. In some forms from the genitive of adder + grass.