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A sexual act of both homo and hetro relationships. Where by the erect penis is inserted into the anus, upon the member becoming flacid, warm urine is expelled into the rectum, creating a hot water bottle effect.
Guaranteed to keep you warm all night.
Phil: How's Debbie?

Alan: She wasn't happy with her Hot Water Bottle. She thought it an inappropriate Valentines day gift.
by Canoesack November 12, 2009