I once heard a rumour (refutation requested) that every word for the Smallest Room, and the actions taken therein, is a euphamism, right back to "bog", which was alleged to be of Roman origin; the source wasn't clear whether that included medical terms.* Calling said room or its porcelain furniture "the Euphemism" sounds about right, which makes it a meta-euphemism.

* "Taking the mickey" has an especially long and interesting etymology.