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.]