A butt (or buttocks).
This term was coined when used in coded languages by the Japanese during World War 2. The code for "buttocks" when decoded and put in English spelt: "bluttis".

Since then it became a fad among British cryptographers and then some Canadians.
Butt = bluttis

Jack: I injured my butt during that heck of a bike ride.
British cryptographer: You injured your bluttis?
by JustARegularGuyCalledAnon April 24, 2008
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