The smallest room in the house
A euphemistic reference to a lavatory.
This euphemistic term is now rather dated and is little used. It began being used in English around the 1930s, as in this example from Archibald Lyall's essay in English etiquette It isn't Done, 1930:
It is all very baffling for the uninitiated foreigner‥who when his host offers to 'show him the geography of the house' finds that his tour begins and ends with the smallest room.
The expression may have been introduced into English from German. Certainly the most celebrated use of it was a quotation from the German musician Max Reger. Reger was notoriously irascible and didn't react well to a savage review by Rudolph Louis in Münchener Neueste Nachrichten, February 1906. His response was:
"Ich sitze in dem kleinsten Zimmer in meinem Hause. Ich habe ihre Kritik vor mir. Im nachsten Augenblick wird sie hinter mir sein"
("I am sitting in the smallest room of my house. I have your review before me. In a moment it will be behind me!")
For those who might need a clearer picture drawn, Reger was suggesting that he was about to use Louis's review as lavatory paper.