It's sort of an improvised slungshot. The real thing is a sailors weapon. I'ts basically a monkey's fist tied out of cordage, with a wrist loop and a lead or brass weight inside the monkey's fist. While slungshots are widely illegal, the beauty of the slungshot is that an improvised model can be kept disassembled and then put together as needed. For example, that lock may be carried in one pocket while a rope/length of chain/sock is carried in another. Assemble it when you're expecting trouble, and then take it apart when you're not. As long as the disassembled components are legal by themselves, you're safe from getting hassled by the cops until it's put together. You might run into trouble if you actually have to use it, but otherwise this minimises the risk of carrying an effective weapon in plain sight.