1. when you can't tell when a person's back stops and their ass begins.
2. the rear part of the human body, extending from the lower end of the ass to the upper back.
3. Otherwise known as Jaack (Jay's Aack). Not to be confused with Jack (JD), Jay's drink of choice.
Kara, Jay's showing his Aack again.
Jay, Get your Aack off the ice.