I’m not sure what the original word actually was as most of today’s words are euphemisms or even, as bathroom, a euphemism to avoid a euphemism. I’m a New Zealander of British stock (parents still speak British and I lived there a while). It comes down to :
toilet – a euphemism based on a woman’s morning ritual, then applied to the room and now to both the room and the bowl
lavatory – means wash room and is a euphemism
loo – from the French for l’eau (water) and is essentially a room with water, a euphemism.
WC or water closet – a room provided with a water source, then applied to the actual apparatus.
bathroom – a room with a bath in the whole world except North America where it now doesn’t need a bath and is used as a euphemism so that Americans don’t have to use another euphemism like toilet or lavatory.
restroom/washroom – variations on bathroom
heads – a boating term used for waterborne craft and naval land institutions, similar to galley (kitchen), brig (jail or cell), wardroom (dining room) etc.
latrines – simple toilet especially in the military, often temporary and little more than a hole with a board above it. Now often used by the Army as a general term.
Then you have the more vulgar, of varying vulgarity, terms such as dunny, bog, john, can, and even more vulgar crpper, shthouse etc. It appears that vulgarity begets accuracy.