When the breasts burst out of tight or undersized clothing, or look like they're about to burst or pop out at any moment. Examples include when the subject is wearing a blouse so small that her breasts cause her blouse to be pushed open between the buttons, or where buttons are popping off or have previously popped off. This could also be used for breasts suddenly and unexpectedly spilling over the top of tight clothing, much to her surprise, delight, dismay, etc.