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.
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   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:

Forbid \For*bid"\ (f[o^]r*b[i^]d"), v. t. [imp. Forbade
   (f[o^]r*b[a^]d"); p. p. Forbidden (f[o^]r*b[i^]d"d'n)
   (Forbid, [Obs.]); p. pr. & vb. n. Forbidding
   (f[o^]r*b[i^]d"d[i^]ng).] [OE. forbeden, AS. forbe['o]dan;
   pref. for- + be['o]dan to bid; akin to D. verbieden, G.
   verbieten, Icel. fyrirbj[=o][eth]a, forbo[eth]a, Sw.
   f["o]rbjuda, Dan. forbyde. See Bid, v. t.]
   1. To command against, or contrary to; to prohibit; to
      interdict.
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            More than I have said . . .
            The leisure and enforcement of the time
            Forbids to dwell upon.                --Shak.
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   2. To deny, exclude from, or warn off, by express command; to
      command not to enter.
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            Have I not forbid her my house?       --Shak.
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   3. To oppose, hinder, or prevent, as if by an effectual
      command; as, an impassable river forbids the approach of
      the army.
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            A blaze of glory that forbids the sight. --Dryden.
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   4. To accurse; to blast. [Obs.]
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            He shall live a man forbid.           --Shak.
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   5. To defy; to challenge. [Obs.] --L. Andrews.

   Syn: To prohibit; interdict; hinder; preclude; withhold;
        restrain; prevent. See Prohibit.
        [1913 Webster]

WordNet (r) 3.0 (2006):

forbidden
    adj 1: excluded from use or mention; "forbidden fruit"; "in our
           house dancing and playing cards were out"; "a taboo
           subject" [syn: forbidden, out(p), prohibited,
           proscribed, taboo, tabu, verboten]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "forbidden":
   banned, barred, contraband, forbade, forbid, illegal, illicit,
   nonpermissible, not permitted, off limits, out of bounds, outlawed,
   prohibited, ruled out, taboo, tabooed, unallowed, unauthorized,
   under the ban, unlawful, unlicensed, unpermissible, unsanctioned,
   untouchable, verboten, vetoed