Shackle

A shackle is a U-shaped piece of metal use to secure something, or a metal loop secured with a quick-release locking pin. The term is sometimes applied to handcuffs.

They are often used in prisons for the inmates. Beast was shackled after he was arrested. The X-Men shackled Sabretooth to a chair but he tricked Jubilee into letting him out.

Black Panther captured Mister Fantastic in shackles for a brief time.