Dog-washing is a sexual position created and made famous by pornstar Tony Eveready. It involves the female laying on her back while vertically projecting her vagina and anus. Then placing both male testicules in the gaping asshole (post anal coitus) and "flipping" the penis to insert it into the vagina effectively creating an epic double-penetration maneuver while requiring only one male.
When Tony was looking to add some spice to his love life he decided to "freak his bitch out" by filling up her "snug asshole" with his "nuts". He was later heard saying "I'm goin' wash your dog..." and dog-washing was created.