The Collaborative International Dictionary of English v.0.48:

Shaddock \Shad"dock\, n. [Said to be so called from a Captain
   Shaddock, who first brought this fruit from the East Indies.]
   (Bot.)
   A tree (Citrus decumana) and its fruit, which is a large
   species of orange; -- called also forbidden fruit, and
   pompelmous.
   [1913 Webster]

WordNet (r) 3.0 (2006):

shaddock
    n 1: southeastern Asian tree producing large fruits resembling
         grapefruits [syn: pomelo, pomelo tree, pummelo,
         shaddock, Citrus maxima, Citrus grandis, Citrus
         decumana]
    2: large pear-shaped fruit similar to grapefruit but with coarse
       dry pulp [syn: pomelo, shaddock]