Source : Webster's Revised Unabridged Dictionary (1913)

Forbidden \For*bid"den\, a.
   Prohibited; interdicted.

         I kniw no spells, use no forbidden arts. --Milton.

   {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.

Source : WordNet®

forbidden fruit
     n : the original forbidden fruit was 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)