A Messy Jessie is any girl who really likes it up the butt and will take it from any guy, anywhere, but also keeps a steady boyfriend who has no idea. A level 2 Messy Jessie is when her boyfriend has heard the stories, but doesn't believe his "little angel" would do that.
Short- MJ, Dirty MJ
Girl 1- That girl has more cum in her ass than a gay porn star.
Girl 2- Yeah, she's a real Messy Jessie.