thass; the area in which your ass and thigh join as one. there should be definition between the two, a thigh... then your ass, creating a "shelf" if you will. but when you start to gain weight you loose that definition, thus creating the infamous thass.
*cousin to the kankle

Thass; the thigh / ass crease or more importantly the lack there of.

where ones ass and thigh look like its merged together

That small area on either a male or females body,that is right between one's ass and one's upper thigh,could be the crease or just that area that on some people just blends together.

The region between your thigh and your ass that pops out to the side when you walk. Usually occurs with girls who have big thighs and a big ass.

Phonetic spelling of the New England pronunciation of "that's"