Chain with a lock on it used for self-defense or assault. Street term. Some say it has to have TWO locks on it, that the two locks leave two marks on the recipient's head that resemble a smiley face. WRONG! Just one lock is used. Originally it was usually a masterlock combination lock that was used, which is round. The round housing of the lock leaves a crescent-shaped mark resembling a smile. The weapon probably originated as a weapon of convenience -- cyclists had them on hand to lock up their bikes and when attacked they grabbed what they had handy. So it's an effective weapon that can't be called weapon by the cops (if you have a bike, or course!).
Some jackass with a knife tried to take my wallet and new bike, so I caught him upside the head with my smiley chain. At least he still has THAT smile on his face.