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)

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "forbidden fruit":
   Eighteenth Amendment, Prohibition Party, Volstead Act, agacerie,
   allure, allurement, ambition, appeal, attraction, attractiveness,
   ban, beguilement, beguiling, bewitchery, bewitchment, blandishment,
   cajolery, captivation, catch, charisma, charm, charmingness,
   come-hither, contraband, dearest wish, denial, desideration,
   desideratum, desire, disallowance, embargo, enchantment,
   enthrallment, enticement, entrapment, exclusion, fascination,
   flirtation, forbiddance, forbidding, glamour, glimmering goal,
   golden vision, hope, index, index expurgatorius,
   index librorum prohibitorum, inducement, inhibition, injunction,
   interdict, interdiction, interdictum, interest, inveiglement,
   invitation, law, lodestone, magnet, magnetism, no-no, plum,
   preclusion, prevention, prize, prohibition, prohibitory injunction,
   proscription, refusal, rejection, repression,
   restrictive covenants, ruling out, seducement, seduction,
   seductiveness, sex appeal, snaring, statute, sumptuary laws,
   suppression, taboo, tantalization, temptation, tree of knowledge,
   trophy, winning ways, winsomeness, wish, witchery, wooing, zoning,
   zoning laws