It is the sad fate of euphemisms that as soon as they become common usage, they need a euphemism in turn. See toilet.

Funny euphemisms, however, often evolve to funny expressions, as in "Ey, f***, mate, I gotta sh**, where's the Wooloomooloo?", which I heard in NSW. (Wooloomooloo is a part of Sydney, the word is used as an extension of "loo", cf. Monty Python.)