The Collaborative International Dictionary of English v.0.48:

Butternut \But"ter*nut`\, n.
   1. (Bot.) An American tree (Juglans cinerea) of the Walnut
      family, and its edible fruit; -- so called from the oil
      contained in the latter. Sometimes called oil nut and
      white walnut.
      [1913 Webster]

   2. (Bot.) The nut of the Caryocar butyrosum and Caryocar
      nuciferum, of S. America; -- called also Souari nut.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oil nut
    n 1: oily drupaceous fruit of rabbitwood [syn: buffalo nut,
         elk nut, oil nut]