If something is sagging, it's drooping down from weight, age, or exhaustion. The sagging ceiling in the kitchen is a clue that there's a leak in the upstairs bathroom plumbing.
If you've got sagging pants, you need a belt or suspenders to hitch them up — or you might want to buy yourself a smaller pair! The adjective sagging is a good way to describe droopy trousers, but it's also useful for an old, sagging table or your sagging spirits at the end of a long, draining day at work. It comes from the verb sag and its Germanic root meaning "to sink."