Longman Dictionary of Contemporary English - taboo

~1 adj 1 taboo subject/area/word a subject etc that people avoid because they think it is offensive or embarrassing  (Death is still a taboo subject to some people.) 2 technical too holy or evil to be touched, or used ~2 n plural taboos 1 a social custom which means a particular activity or subject must be avoided  (about/on/against)  (a strong taboo against fighting in public) 2 a religious custom that forbids a particular activity because it may offend God