buck·bean 
(bŭkbēn′)
(bŭkbēn′)n.
A perennial herb (Menyanthes trifoliata) native to the Northern Hemisphere and having trifoliate leaves and clusters of white, pink, or purplish flowers.
[Translation of Flemish bocks boonen, goat's beans : bocks, genitive of bock, goat + boonen, pl. of boon, bean.]