A plump, luscious, spectacular, round buttox belonging to a female. This female typically has a heavier build, and has thick, muscular (or cellulite-filled) thighs that accompany the jenny butt. The girl might be considered a bit on the chubbier side, but that's what makes the jenny butt so lovely! More cushion for the pushin'.
Observer 1: "Godddddaaaaamn! You see that girl's ass?"
Observer 2: "Fuuck yeah, I did! She's got a jenny butt!"