Edit
Supporting or lifting the bottom edge of the butt cheeks.
Used when something (usually panties, but also pants, shorts, bikini bottoms, hands, the edge of something, etc.) is snugly pressed against or just above the point where the thighs meet/transition into the ass. The pressure being placed on this point causes the butt to be lifted slightly and to spill out a little over the edge of the item in that place, similar to the effect a corset has on a woman's bustline.
Prime examples can be seen in post #325675, post #2194153, and post #1800402.
This tag may still be used even if only one side of the ass is being fully supported. However, the half in question should unquestionably be a prime example of ass support.
This should NOT be confused with something simply pressing against the ass as it rides low to mid-level on it (post #2112035, post #2206991, post #2193221), or pulled down just slightly past it (post #2196070, post #2192644). Something like post #2203808 would almost qualify for this tag, but the panties are just being pulled down, and as a result are not pressed tightly against the body enough to create the effect described earlier.