When you are fucking a girl from behind, either in the ass or pussy, and you are about to cum you pull out and shoot it between her butt cheeks. You then grab said cheeks and push them together and apart really fast.
Dude, can I sleep on yer couch? I gave my wife a Melky Cabrera.
To perform a sexual act on another individual on public or group transport (such as a bus, train or plane). Sometimes but not always by a girl named Melissa.
"I fully saw this girl give this guy a Melsy on the train the other day"
or
"We were all so drunk on the way back from the wedding that Mel gave James a full 'Melsy' in front of everyone"