The Collaborative International Dictionary of English v.0.48:
Groundnut \Ground"nut`\ (ground"n[u^]t`), n. (Bot.)
(a) The fruit of the Arachis hypog[ae]a (native country
uncertain); the peanut; the earthnut.
(b) A leguminous, twining plant (Apios tuberosa), producing
clusters of dark purple flowers and having a root
tuberous and pleasant to the taste.
(c) The dwarf ginseng (Aralia trifolia). [U. S.] --Gray.
(d) A European plant of the genus Bunium (B. flexuosum),
having an edible root of a globular shape and sweet,
aromatic taste; -- called also earthnut, earth
chestnut, hawknut, and pignut.