Fulldin Butt (Pronounced Full-Din- Butt) Is a butt, normally a female's butt, where the lower-half sticks out, then folds back up unto the waist. Much in the way a beer belly hangs over the front of a pair of pants. Examples include: Jessica Alba, Jamie Pressly, and Roselyn Sanchez. Not to be confused with a Bubble-Butt, which is just round and sticks out like a bubble. Or a Flat-Butt, which just extends out slightly, but does not fold underneath.
I saw this really skinny chick with a Fulldin Butt. I couldn't believe someone so small could have such a thick ass!