The proverb suggests that persons seemingly weak or in poor health often live a long time. This is mostly used for very sick people that are expected to die very soon, but they go on to live with their sickness for a long time.
It can also be used to suggest that an issue that is lingering is likely to take a long time to resolve.
Another variant of this proverb is: A creaking door hangs long on its hinge.
— I’m worried that my grandfather may not live much longer. He’s been sick for so many years.
— Well, if it’s any comfort, I’ve heard that a creaking door hangs long.