When you use your elastic waistband pants as your shirt, bra, and pants.
Sue was getting dressed for a trip to wal-mart and all she had were a pair of elastic waistband pants. No shirt or bra, just pants. So she put them on and pulled them up just enough to cover her floppy boobs, turning her pants into a pair of shirbrants. See "people of wal-mart" for picture example.