The Collaborative International Dictionary of English v.0.48:

Forbidden \For*bid"den\, a.
   Prohibited; interdicted.
   [1913 Webster]

         I know no spells, use no forbidden arts. --Milton.
   [1913 Webster]

   Forbidden fruit.
   (a) Any coveted unlawful pleasure, -- so called with
       reference to the forbidden fruit of the Garden of Eden.
   (b) (Bot.) A small variety of shaddock (Citrus decumana).
       The name is given in different places to several
       varieties of Citrus fruits.
       [1913 Webster]

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):

forbidden fruit
    n 1: originally an apple from the tree of knowledge of good and
         evil in the Garden of Eden; it is now used to refer to
         anything that is tempting but dangerous (as sexuality)