Refers to when a full-grown man pulls his pants to the ground like a toddler while using a public urinal.
Typically at stadium or large crowded bathroom.
"That browns is going full butt in there"
Or news headline
"Bills fan goes full butt in Rival stadium restroom"
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!