From the movie The Forty Year-Old Virgin.
This is the official definition from the director/co-writer, Judd Apatow.
You shit on someone's chest (or someone shits on yours) and you smush it all around with your ass cheeks, leaving a bow-tie shaped shit-smear
Having run out of disgusting, non-erotic things to do, I decided to give her a cincinatti bow-tie. It didn't really turn me on, and I'm pretty sure she hated it, but it did sort of look like a bow-tie... so that was neat.