A butt so fat that it seems like a drink could rest on top of it while the person is standing. Can be a compliment or used derogatorily. Not a term that is thrown around for just anyone, only a select few actually have shelf booties. Can also be used as a proper name when referring to a specific person with a shelf booty.
1) "Mmm that girl got a shelf booty, doe, and I'm tryna holler
2) "Damn now THAT'S a shelf booty. That ass is tooo fat!"
3) "I'm not doing anything tonight, I might just see what shelf booty is up to."
a girl you keep on the side
you might have a girlfriend and then will refer to your side jont
"I'm going out tonight with my side jont so keep it dl and don't let my girlfriend find out."