A brank is a metal device that is used to restrain the tongue and secured by a frame around the head. It was used in the past as a torture device.
More About Brank
Branks were traditionally used to restrain women who were particularly quarrelsome and prone to argue. The device is the human equivalent of a horses bit. It is meant to be uncomfortable and restrain the individual from speaking. It was used on women who were found guilty of being too noisy. Today the device is generally only used in the BDSM community as a restraint on a submissive.