The Collaborative International Dictionary of English v.0.48:

mockernut \mockernut\, mocker nut \mocker nut\n.
   1. (Bot.), A smooth-barked North American hickory (Carya
      tomentosa) with 7 to 9 leaflets bearing a hard-shelled
      edible nut, which is far inferior to the true shagbark
      hickory nut.

   Syn: mockernut hickory, black hickory, white-heart hickory,
        big-bud hickory, Carya tomentosa .
        [1913 Webster + WordNet 1.5]

   2. The fruit of the mockernut[1].
      [PJC]

WordNet (r) 3.0 (2006):

Carya tomentosa
    n 1: smooth-barked North American hickory with 7 to 9 leaflets
         bearing a hard-shelled edible nut [syn: mockernut,
         mockernut hickory, black hickory, white-heart
         hickory, big-bud hickory, Carya tomentosa]