Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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