Any form of explosion just inside the anus. Could be the result of inserting and igniting gasoline or gunpowder, attempting to light a fart, or that Mexican food from yesterday. Depending on the strength of the sphincter, severe anal combustion may result in blowing out the anal socket or rupturing of the rectum.
I had pretty bad anal combustion after that spicy 15 bean soup--it almost blew out my sphincter!