Boston Backslap
The Boston Backslap is the sexual act of rooting doggy-style and the male pretends to be edging near to climax, instead shitting into his hand and slapping it on the females back.
Billy: Bro I was fucking that dog of a cunt Deanna, and she was a shit root so I shit in my hand and slapped it on her back.

Joey: So you Boston Backslapped her cunt?

Billy: Yeah, I only found out after she was your mrs though....