The phrase "butt hole" is a slang term that refers to the opening of the rectum, the part of the body where solid waste comes out during bowel movements.
I get tired of rubbing its little butt hole back there. (cheersandgears.com)
@pattic: I grew up in a Christian environment too and it was the deepest, blackest, butt hole of rage, anger and despair I have ever experienced. (religion.blogs.cnn.com)