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]

The Collaborative International Dictionary of English v.0.48:

Tabu \Ta*bu"\, n. & v.
   See Taboo.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tabu
    adj 1: forbidden to profane use especially in South Pacific
           islands [syn: taboo, tabu]
    2: 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]
    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]