When a female's ass is so big that it extends over her sides so much that one can observe it from the front. While seated, it can be grabbed from the side. It is a little bit lower than where the widest part of the hips should be. Depending on the male, it can be desirable or unappealing. Never comment on a woman's side booty. Ever. You won't get laid.
Male: I saw that smokin' hot pic of you on facebook. You have a great ass.
Female: What the fuck, how can you know that..it was from the front view.
Male: Yea, but I could see your side booty.
Female: I hate you