A sexual act similar to the "Dirty Sanchez." While performing in the doggie style position, the man puts his index finger in the woman's anal cavity. He then removes his finger from her anus and rather than wipe it across her upper lip as in the dirty sanchez, he puts it in her ear (right or left depending on which hand he favors). The man then twists his finger as if he were performing a "wet willy" the common childhood prank.
Instead of giving her a shit mustache, he opted to give her the Dirty Willy Sanchez, putting his index finger in her ear.