1. [n]  an inhibition or ban resulting from social custom or emotional aversion
2. [n]  a prejudice (especially in Polynesia and other South Pacific islands) that prohibits the use or mention of something because of its sacred nature
3. [adj]  excluded from use or mention; "forbidden fruit"; "in our house dancing and playing cards were out"; "a taboo subject"
4. [adj]  forbidden to profane use especially in South Pacific islands

TABU is a 4 letter word that starts with T.