Similar to bursting breasts, bursting ass is used when some degree of wardrobe malfunction is caused due to the clothing being too small for the ass in question. That, or the clothes appear small/tight enough to be about to “burst” off of the ass. This tag is not to be used just because the clothing over the ass (like shorts or pants, for example) are torn. It should be apparent that the wardrobe malfunction is caused primarily by the butt being too big for whatever clothing the character is wearing.
