I have come across the usage of 'throne' for a lavatory. Is there any special etymology to this? Is it simply because a throne is a seat? Or does the equivalence have any royal *under*pinnings to it? ;)
According to Etymonline, this meaning of throne is recorded from 1922.
As to why the word throne acquired this meaning, there is a theory. Back in the European Middle Ages it was only the powerful rulers of an area who had what we today would consider toilets. There was an isolated room in the castle where only the master of the premises had access. Since this treatment was fit only for a king (or someone who wielded power that resembled that of a king), the seat where the person was allowed to sit was compared to a throne, where only one person was allowed to sit as well. Hence the association.
We also call the toilet trono (throne) in Spanish and according to other comments it's also called that way in other languages such as German and Polish (I'd dare say it's the same for almost every other language too). As for its origin I think it's easy to see the relation. It´s just a humourous way to refer to the toilet, giving the idea that the person there is doing something important, when in fact he is most probably not.
Do a Google ngram viewer search, when interested to find out about etymology. Google has been digitizing a lot of books dating from 1500s (at least - I am sure).
I have the following evidence to say the usage of throne as a seat of power is dated much back.
the above book is the oldest reference to any book containing the word throne within Google Book's database dating from 1500 AD.
you can also see ngram usage result for the word throne here: http://books.google.com/ngrams/graph?content=throne&year_start=1000&year_end=2000&corpus=0&smoothing=3
As far as the investigation for of the word with regard to lavatory (using ngram), I am afraid one will have to read a lot :) of Google book results..
I just spent some time reading the titles.. most of the titles were talking about God or King and were written in context of events from history.. And we already know that older books were more conservative about the choice & usage of the words.
We now, can make fun of the word throne because we are not ruled by kings any more. I doubt whether one would have used the word earlier than 1800/1900s with regard to toilet.
May be as Irene said, Etymonline.com is telling us the correct date with the lavatory sense of usage :)