When a womans ass starts to increase in size, but instead of increasing to just be big and nasty, it does something else.
An Ass Bag retains it's upper dimensions, but increases down the leg. It appears to grow out of the upper hamstring region.
To make sure you've seen a true ass bag, just watch it move. You will see the normal ass pieces move accordingly. But the bag will show up in the last portion of the stride(look at the lower butt where it meets the hamstring) as what can only be called...Ass Bags.
The cause of an ass bag is actually the ass starting to lose it's rounded shape. It forms a tear drop and starts dropping down the legs.