The word throne is simply a euphemism for toilet, and we have plenty of them for subjects that are considered unmentionable.
There are many more names for the toilet, such as: hopper, Johnny, janey, loo, potty, or privy.
This particular euphemism, throne, is said to have come from none other than King Louis XIV, who often saw to royal business from his toilet, deeming it an alternative to his usual throne.
Early water flushing toilets were found at Skara Brae in Orkney, Scotland, which date from about 3100 B.C. until 2500 B.C.