A dirty Sanchez is a sexual act that occurs during anal play, such as anal sex or anal fingering, in which feces, either from the penis or a finger, are wiped on the receiving partner's upper lip. The act was dismissed as an urban legend in "Dirty Words: A Literary Encyclopedia of Sex." Sex advice columnist Dan Savage also called it "completely fictional" in a 2003 column. The term also has a racist connotation that many find more offensive than the term itself.
Even so, the term has made its way into popular culture and popular imagination, so it is likely that more than a few people have experimented with this practice in real life. This would be considered a form of scat play.