The term lavatory, or lav, derives from the Latin, which in turn comes from Latin, to wash. It used to refer to a vessel for washing, such as a sink or wash basin, and so came to mean a room with washing vessels. Since these rooms often also contain toilets, the meaning evolved into its current meaning, namely the polite and formal euphemism for a toilet and the room containing it. Lavatory is the common signage for toilets on commercial airlines around the world.
It's etimology is obscure, but it might derive from the word Waterloo.Other theories are:
- That it derives from the term "gardyloo" (a corruption of the French phrase gardez l'eau loosely translated as "watch out for the water!") which was used in medieval Edinburgh when chamber pots were emptied from a window onto the street. However the first recorded usage of "loo" comes long after this term became obsolete.
- That the word comes from nautical terminology, loo being an old-fashioned word for lee. The standard nautical pronunciation (in British English) of leeward is looward. Early ships were not fitted with toilets but the crew would urinate over the side of the vessel. However it was important to use the leeward side. Using the windward side would result in the urine blown back on board: hence the phrases 'pissing into the wind' and 'spitting into the wind'. Even now most yachtsmen refer to the loo rather than the heads.
- That an early British toilet manufacturer produced a model of cistern named "Waterloo" (in honour of the Battle of Waterloo), and the term derives from 'going to the Waterloo', and then abbreviated to simply as 'going to the `loo'.
The WC is the initial letters of Water Closet, surprisingly used commonly in France (pronounced "le vay-say" or "le vater"), the Netherlands (pronounced "vaysay") and in Germany (pronounced "veh-tsay"). In Mexico, WC is very common everywhere on public toilets, although the majority of the people there do not know the meaning of the 'mysterious' letters on the door.
In Tudor England a privy was first referred to as a jakes in 1530. In modern Ireland the cognate term jacks is still used, and is a very common method of referring to the toilet.
House of Office
The "house of office" was a common name for a toilet in seventeenth century England, used by Samuel Pepys among others.
Lexicographer Eric Partridge derives khazi, also spelt karzy, kharsie or carzey, from a low Cockney word carsey originating in the late 19th century and meaning a privvy. Carsey also referred to a den or brothel. It is presumably derived from the Italian casa for house, with the spelling influenced by similar sound to khaki. Khazi is now most commonly used in the city of Liverpool in the UK, away from its cockney slang roots.
The bog is a colloquial expression in British English for a toilet. Originally "bog" was used to describe an open cesspit and the word was later applied to the privy connected to it. More wide-spread is the usage bogroll, meaning toilet paper. See also tree bog, not to be confused with the swampland meaning of bog.
The Dunny is an Australian expression for an outside toilet. The person who appeared weekly to empty the pan beneath the seat was known as the dunnyman. The word derives from the British dialect word dunnekin, meaning dung-house.  It is now an informal word used for any lavatory and is most often used referring to drop or pit lavatories in the Australian bush.
The Netty is a Northern English Expression for an outside toilet.
The Shithouse is British and American slang for the toilet.
The John is an American term for the toilet.
The Privy is an old fashioned term used more in the North of England and in Scotland, a possible derivation meaning private place.
The Crapper is another term in general use, along with the word 'crap', meaning excrement. Crapper is the name of one Thomas Crapper, who is mistakenly associated with the invention of the modern flush toilet. He did have several patents related to plumbing, but the word "crap" predates him.
Vin is used by some members of the English Aristocracy and upper classes. Although the word 'vin' is rarely used in modern England, some private schools in England still use the term, such as Summer Fields School a Prep School in Oxford. In that school, it is against the rules to refer to the lavatory as 'the vin' when asking for permission to leave the room to relieve one's self.
Latrine is a term common in the US Military, specifically for the Army and Air Force for any point of entry facility where human waste is disposed of, which a civilian might call a bathroom or toilet, regardless of how modern or primitive it is.
In Philippine English and some other countries, "Comfort Room", or "C.R.", is the most common term in use.