Lack of definition between the bottom and thigh. The appearance of bottom and thigh merging into one shapeless mass. Typically this condition is most noticeable when the subject wears jeans.
Not from behind. She's got a thighbum.
Oh my god - it's like two straws connected to her back.
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Does my bum look good in these jeans?
No sweetie. It's like two pancakes vacuum-packed in denim.
Not my thighbum!!