Definitions for buck bean
Random House Webster's College Dictionary
a bog plant, Menyanthes trifoliata, of the gentian family, having narrow clusters of white or pink flowers.
Origin of buck bean:
a plant (Menyanthes trifoliata) which grows in moist and boggy places, having racemes of white or reddish flowers and intensely bitter leaves, sometimes used in medicine; marsh trefoil; -- called also bog bean