What is "buck bean"

buckbean \buckbean\, Buck bean \Buck" bean`\(b[=e]n`). (Bot.) A perennial plant ( Menyanthes trifoliata) of Europe and America which grows in moist and boggy places, having racemes of white, reddish, or purplish flowers and intensely bitter trifoliate leaves, sometimes used in medicine; marsh trefoil; -- called also bog bean. It often roots at the water margin and spreads across the surface.

Syn: water shamrock, bogbean, bog myrtle, marsh trefoil, Menyanthes trifoliata.