Someone who's bony is very thin. Your cat may have been plump when he was young, and become bony as he got older.
If you're so skinny that your bones are prominent, your friends might call you bony and encourage you to eat more. Another way to use the adjective bony is to mean "like bone" or "containing bone." Some animals have bony shells or protective plates — and you might not enjoy your fish dinner if it's way too bony. Bony shares a root with bone, the Old English ban, "bone or tusk."