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1) Usually related to overseas expats, stenning refers to a drunk (British) man who follows girls to the bathroom and enjoys watching them pee whilst he himself urinates in his own trousers.

2)stenning can also be used as an adjective to describe a small, balding man/woman who is working as a language teaching in a foreign country and enjoys wearing an old persons underwear under his clothes whilst in class.
My teacher is a right stenning....
by Don Oso June 14, 2010