Also known as: 見せTバック, 見せハイレグパンツ, 鲸尾(内裤)
When the top of one's Thong can be seen over the rear waistline of the pants, forming what looks like a whale's tail.
For a literal whale's tail, see Cetacean Tail. See Panty Straps for when the straps are visible from the front.