Taboo
In a group or society, a taboo is something that is not acceptable to talk about or do.  An activity or behavior can be taboo in one culture, but not in another.  Some things like cannibalism and sexual relationships between siblings are taboo in almost all societies.  Sometimes even talking about taboos is taboo.  Some taboos may also be against the law, and people who break them may be punished. Breaking taboos can seem rude, and can cause embarrassment or shame.