The Collaborative International Dictionary of English v.0.48:

Taboo \Ta*boo"\, a. [Written also tabu and tapu.]
   [Polynesian tabu, tapu, sacred, under restriction, a
   prohibition.]
   Set apart or sacred by religious custom among certain races
   of Polynesia, New Zealand, etc., and forbidden to certain
   persons or uses; hence, prohibited under severe penalties;
   interdicted; as, food, places, words, customs, etc., may be
   taboo.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Taboo \Ta*boo"\, n.
   A total prohibition of intercourse with, use of, or approach
   to, a given person or thing under pain of death, -- an
   interdict of religious origin and authority, formerly common
   in the islands of Polynesia; interdiction. [Written also
   tabu.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Taboo \Ta*boo"\, v. t. [imp. & p. p. Tabooed; p. pr. & vb. n.
   Tabooing.]
   To put under taboo; to forbid, or to forbid the use of; to
   interdict approach to, or use of; as, to taboo the ground set
   apart as a sanctuary for criminals. [Written also tabu.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

taboo
    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]
    2: forbidden to profane use especially in South Pacific islands
       [syn: taboo, tabu]
    n 1: a prejudice (especially in Polynesia and other South
         Pacific islands) that prohibits the use or mention of
         something because of its sacred nature [syn: taboo,
         tabu]
    2: an inhibition or ban resulting from social custom or
       emotional aversion [syn: taboo, tabu]
    v 1: declare as sacred and forbidden

Moby Thesaurus II by Grady Ward, 1.0:

136 Moby Thesaurus words for "taboo":
   Eighteenth Amendment, Prohibition Party, Volstead Act, anathema,
   ban, banned, bar, bar out, barred, barring, blockade, boycott,
   censorable, censored, circumscription, contraband, count out,
   cut off, debar, debarment, debarring, demarcation, denial, deny,
   dirty, disallow, disallowance, embargo, enjoin, exception, exclude,
   exclude from, exclusion, explicit, forbade, forbid, forbiddance,
   forbidden, forbidden fruit, forbidding, freeze out, ignore,
   illegal, illicit, impolite, inadmissibility, indecorous, index,
   index expurgatorius, index librorum prohibitorum, inhibit,
   inhibition, injunction, interdict, interdicted, interdiction,
   interdictum, jargonal, jargonish, keep out, law, leave out,
   limitation, lock out, lockout, narrowing, no-no, nonadmission,
   nonpermissible, not permitted, off limits, omission, omit,
   ostracize, out of bounds, outlaw, outlawed, pass over, preclude,
   preclusion, prevent, prevention, prohibit, prohibited, prohibition,
   prohibitory injunction, proscribe, proscribed, proscription,
   refusal, refuse, regulation, reject, rejection, relegate,
   relegation, repress, repression, repudiate, repudiation,
   reservation, restraint, restricted, restriction,
   restrictive covenants, rude, rule out, ruled out, ruling out,
   sanction, say no to, scatological, send to Coventry, shut out,
   slang, slangy, statute, sumptuary laws, suppress, suppression,
   tabooed, unacceptable, unallowed, unauthorized, under the ban,
   unlawful, unlicensed, unmentionable, unpermissible, unsanctioned,
   unspeakable, untouchable, verboten, vetoed, zoning, zoning laws