Also known as: 下臀, underbum
When the bottom edge of the butt cheeks is visible beneath the clothes.
Underbutt is the equivalent of Underboob for butts. It can happen when a character wears a short Skirt, Dress or Short Shorts/Micro Shorts that shows only the lower portion of their ass cheeks. The exposed area may still be covered in Pantyhose or the like.