If a police officer has to manacle your hands behind your back, you're in big trouble. That's just a fancy way of saying that you've been handcuffed.
Used as a noun, manacle is a synonym for shackle, meaning "a metal chain or band, used to fasten someone’s hands or ankles together." You’re more likely to see the noun form of this word in its plural form manacles, since — like socks or mittens — a pair is usually required. (Quite unlike socks or mittens, manacles are not at all comfortable or pleasant to wear.) You can say that someone who has been restrained using manacles has been manacled.