Slang word used in BDSM circles, to describe a form of restraint, which uses metal, rather than more pliable options such as leather, straps, rope, etc.
Many prefer to experience the 'authentic' bondage of ancient times, hence the use of more less pliable devices for restraining their subjects.
This includes manacles, or metal restraints, such as handcuffs and even leg irons.
There are speciality shops, that cater to providing such devices, on a custom basis.
Caution should be taken, when using such devices, as serious and permanent injury can occur, when improperly used, or over a long period of time.
Risk of injury can occur due to the restraints having no give to them, no leeway given.