back passage 
noun British, euphemistic
The rectum.If you have bleeding from the back passage or any swellings around the opening to the bowel (the anus), see your GP....
- Readers may recall the legendary story of the 20-year-old Taiwanese woman who had to have a mobile phone surgically removed from her back passage after a sex game went horribly wrong.
- She flipped me over, pulled down my bikini and without any word of an excuse me or a shake of the hand she had stuck a thermometer up my back passage.