Dictionary:

buckbean

  (bŭk'bēn')

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.]

The noun has one meaning:

Meaning #1: perennial plant of Europe and America having racemes of white or purplish flowers and intensely bitter trifoliate leaves; often rooting at water margin and spreading across the surface
  Synonyms: water shamrock, bogbean, bog myrtle, marsh trefoil, Menyanthes trifoliata