The Collaborative International Dictionary of English v.0.48:

Bitternut \Bit"ter*nut"\, n. (Bot.)
   The swamp hickory (Carya amara). Its thin-shelled nuts are
   bitter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bitternut
    n 1: hickory of the eastern United States having a leaves with 7
         or 9 leaflets and thin-shelled very bitter nuts [syn:
         bitternut, bitternut hickory, bitter hickory, bitter
         pignut, swamp hickory, Carya cordiformis]