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.
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pignut \Pig"nut\ (p[i^]g"n[u^]t), n. (Bot.)
   (a) See Groundnut
   (d) .
   (b) The bitter-flavored nut of a species of hickory (Carya
       glabra syn. Carya porcina); also, the tree itself.
       [1913 Webster]

WordNet (r) 3.0 (2006):

pignut
    n 1: an American hickory tree having bitter nuts [syn: pignut,
         pignut hickory, brown hickory, black hickory, Carya
         glabra]