When a girls ass is so tight/perfect/bubbly that it reminds you of Rihanna's ass. The kind of ass that when she is on all fours you will lick from her clit to the top of her ass crack and love every second of it.
"Man, I love that girl's ass. It's perfect! She's got that riri booty. "
Girl 1: Hey did you have sex with that guy last night?!
Girl 2: No, all he did was call me to make out. It was like a PG rated booty call.
Girl 1: How lame...
it refers to the buttocks of a person and how large it may be. depending on the tone of the person saying it, it usally means a compliment from a man to a woman and an inslut from a woman to man or woman.
"dam son that chick got a big booty!"
a man refering to another man about a woman's buttocks. (comliment)
"Look girl at Lora's big booty. It disgusting."
a woman refering to another woman about a woman's buttocks.