As best I remember, this is the derivation of Loo.
Before sanitation in arrived in Britain, some time around the 15th century, it was common to go to the toilet in a bucket and throw the effluent out onto the streets from the windows of your house afterwards - including from upstairs.
As you can imagine, this wouldn't be great for anyone walking in the street below.
In Edinburgh, as a warning to anyone passing in the street people would shout "Gare de l'eau!" just before they threw out the bucket of effluent, and I mean just before
If you didn't see where it was coming from and get out of the way you were soaked anyway!!! Gare de l'eau obviously comes from the French meaning watch out for the water. The peasant masses weren't too hot on their French, and the phrase colloquially was "Gardyloo!"
And that's where the word "loo" comes from.
...The things you learn on night walking tours round Edinburgh
And if anyone wants to check, I'm reliably informed "Gardyloo" is in the Oxford English Dictionary....