Unmentionables

It is rude to make mention of one’s unmentionables.
This creates a rather interesting dilemma: what is an unmentionable?
If you cannot mention then, then how does one know what not to mention?
I heard about a Downtown club where they’d mention the unmentionables.
Knock twice, then once, and then three times.
The password is “They forgot to mention it to me.”
The cops used to raid the place all the time, but these days they just collect a bribe and move on.
It’s hardly worth mentioning, really.
Perhaps that’s why the unmentionables are unmentionable.
Forget I ever mentioned them.