Secret code for masturbation.
"What are you going to do tonight?"
"Uh, you know... I'm gonna go Wash the Dog."
What you do when there has been a family row and you slip away.
My sister and husband were having a rough time so I slipped away to wash the dog.
The act of doing/snorting cocaine. A clever way to hide it from a large crowd but also keep humor high.
“ Hey you guys, wanna go wash your dog?”
“Hey Chris where have you been?” “Sorry I was in the bathroom, I had to wash my dog”