To make a simple earth-closet, you dug a hole in the ground, leaving the earth piled beside the hole. You could simply squat over the hole, but it was common to build a seat above it. After each visit you shovelled in a little earth on top of the excrement. This process continued until the hole was full, when you covered it over, and dug a new hole a short distance away.
Using such euphemisms seems to be general human behaviour; similar expressions crop up in many cultures and languages; see astronauts, Australia, Bible, France, Germany, New Zealand, trains, United States.
Hamilton Ellis suggests that railway companies were responsible for the use of the word toilet. Eighty years ago some railway carriages had a room in which to wash, labelled 'Toilet', opposite one with a water-closet, labelled 'WC'. When the two rooms were combined, the 'Toilet' label was used.
In the sanitaryware business a 'lavatory' means a wash-basin; a recepticle for excretion is called a 'closet'.
For the verb 'go to the lavatory' we use these euphemisms: explore
the geography of the house, go to the
bathroom/
When asked to give a word of advice to newly-commissioned officers, the
Duke of Wellington is supposed to have said 'Never neglect an
opportunity to pump ship!'.