a girl with a nice ass who upon getting married or becoming involved in a relationship with a man finds that the size of her ass begins to multiply in a rate that corresponds with the period of the relationship. (Similar to the way rabbits multiply when they "couple.")
Peter: That Jane used to be hot, but since she hooked up with Mike, damn her ass has got huuge!
Freddy: She's a rabbit-ass. Since she's been with him, her ass is multiplyin' in size every week!