The act of having rough anal sex with someone's mouth, which usually results in ripping of the lining of the roof of the reciever's mouth. It is generally a rough but very sensual act, most likely followed by ejaculation into ones mouth. If performed correctly the ejaculate tends to leak out of the nostrils; not to be confused with a "candy cane." There are two possible ways to begin the act of mouth buttsex. 1. Start by ramming the sphincter of the proposed receiver. 2. Being you start the oral buttsex flaccid, as the erection occurs you begin to ram the receivers mouth as roughly as non-fatally possible. Circular motions of thrusting are encouraged during the act. If the "thruster" can maintain an erection after ejaculation into ones mouth, it is common courtesy to proceed with normal anal sex following the oral buttsex. As is "making out." As with wine it is usually an acquired taste, but once approved by both parties, it becomes very pleasurable for all involved, and has potential to bring the participants closer than ever during, and after the act has finished.
"Dude Kelly totally let me have mouth buttsex with her last night, I was thinking about breaking up with her, but tomorrow we are going to pick out some engagement cock rings"