A common euphemism for the toilet in the spoken Welsh of north Wales is "lle chwech", literally "six place" ("chwech" being "six" in Welsh). Note this refers mainly to the room rather than the porcelain throne itself.
Geiriadur Prifysgol Cymru (the Welsh equivalent of the Oxford English Dictionary) states here that this is a borrowing from the English slang six "a privy".
There is a reference to six with this meaning in The Routledge Dictionary of Historical Slang here. It states:
six A privy: Oxford University: ca 1870-1915. ?origin
I'm not sure to which Oxford publication the date range above refers. I've looked in the New English Dictionary … under six and found nothing relevant.
In conversation, someone has suggested a link to the French "sis" from "soeir" meaning "to sit", which seems plausible semantically but I'm no expert on French.
Another possibility is that it may have cost sixpence to visit a toilet at some point. However, this seems expensive considering "spend a penny" is apparently from the 1850s and that it was only 2p to visit the lavatory in 1977 according to A Dictionary of Slang and Unconventional English.
Any thoughts on the origin of six in this context?
Best Answer
The best reference I could find is from British Library Sounds web page:
Another possible explanation I've found is that lle chwech (which translates to "six place") refers to the workers toilets which commonly have six seats. I've found this explanation in three different sites (two of them are answers in a forum and one of them is a comment in a blog page). The people who provided the explanation appear to be of Welsh origin.
Furthermore, I've found a supporting evidence for the second theory in an archaeology book written in Welsh. It provides a reference to a six-seater privy in a quarry in north Wales. Here is the excerpt and the image from the book Llechi Cymru: Archaeoleg a Hanes (by David Gwyn):