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]

The Collaborative International Dictionary of English v.0.48:

Hickory \Hick"o*ry\, n. [North American Indian pawcohiccora
   (Capt. J. Smith) a kind of milk or oily liquor pressed from
   pounded hickory nuts. "Pohickory" is named in a list of
   Virginia trees, in 1653, and this was finally shortened to
   "hickory." --J. H. Trumbull.] (Bot.)
   An American tree of the genus Carya, of which there are
   several species. The shagbark is the Carya alba, and has a
   very rough bark; it affords the hickory nut of the markets.
   The pignut, or brown hickory, is the Carya glabra. The
   swamp hickory is Carya amara, having a nut whose shell is
   very thin and the kernel bitter.
   [1913 Webster]

   Hickory shad. (Zool.)
   (a) The mattowacca, or fall herring.
   (b) The gizzard shad.
       [1913 Webster]

WordNet (r) 3.0 (2006):

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