portable night pots carried in ladies’ muffs

At the OUPBlog, Anatoly Liberman looks at the etymology of the word "loo":

French l’eau “the water” has been offered as the etymon (source) of loo. In the 19th century, it was common to invent French etymons of English words and explain the discrepancy in the pronunciation as the result of English speakers’ inability to produce foreign sounds. But surely, law or low are more accurate renderings of l’eau than loo. And why should loo have been borrowed from French, a language that lacks its equivalent? Engl. ablution contains the syllable loo, and so does Latin ablui “I have washed off.” If loo originated at Oxford and Cambridge, bookish word play could be considered. However, as noted, we do not know the social stratum in which loo was coined.

Since the abbreviation for water closet is WC, loo resembles the second half of double u, but loo, definitely not an “Americanism,” has never been pronounced like British lew (that is, with the sounds of few, pew, cue) even in the United States, let alone England or Scotland. The same objection holds for the attempts to derive loo from lee “shelter,” reportedly pronounced somewhere as lew, and from the phrase in lieu of. Lavatories are often named for people. Engl. john is a classic example; its predecessor was jakes. In Germany, Tante Lotte “Aunt Lotte” and many other similar names have been attested. It would be nice if Uncle Lou, the eponymous ancestor of British toilets, were discovered, but so far he has not turned up. According to one version, during a house party in Ireland, about 120 years ago, Lady Louisa Anson’s two younger sons put her name card on the guest lavatory. Henceforward, highbrows (or so we are informed) talked of going to Lady Loo. Perhaps they did, but, to the best of our knowledge, this piece of aristocratic slang has left no trace in any story or newspaper article, and no member of the British nobility seems to remember it. We may forget it too.