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]