Because the person who invented the toilet were named John Harrington.The inventor of the flushable toilet was named Sir John Harrington. Hence the term "john."
'Bog' is a slang term sometimes used in Britain for a toilet.
An old (but still used elsewhere) term for public rest room, washroom, bathroom.
John Crapper invented the first toilet.
The term "water-closet" was an early term for a room with a toilet http://en.wikipedia.org/wiki/Flush_toilet
A biffy is a term used in the United States for a toilet or an outhouse.
A big white telephone is a slang term for a toilet, especially when used as a receptacle in which to vomit.
The toilet was invented by Sir John Harington in 1590.
it was the first flush toilet
the inventor of the toilet
A 'john' is slang for both a prostitute's customer and a toilet.