Courtesy of the Free Online dictionary

ta·boo also ta·bu (t-b, t-)
n. pl. ta·boos also ta·bus
1. A ban or an inhibition resulting from social custom or emotional aversion.
2.
a. A prohibition, especially in Polynesia and other South Pacific islands, excluding something from use, approach, or mention because of its sacred and inviolable nature.
b. An object, a word, or an act protected by such a prohibition.
adj.
Excluded or forbidden from use, approach, or mention: a taboo subject.
tr.v. ta·booed also ta·bued, ta·boo·ing also ta·bu·ing, ta·boos also ta·bus
To exclude from use, approach, or mention; place under taboo.