You can't say that

Some words are banned from public conversation because they are considered too coarse and offensive. And there are some true things that we aren't allowed to say out loud; you can probably think of a few examples.

Suppose you had the power to take a previously innocuous word and temporarily add it to the list of forbidden swear words. Or suppose you had the power to declare one specific subject area off-limits. What is one thing you might add to the list of things that no one is allowed to say?