A boot knife or a gambler's dagger is a small fixed-blade knife (usually, a dagger) that is designed to be carried in or on a boot. All Judges carry a boot knife as a backup weapon. Anderson's was seen in the 2012 Dredd film.
Typically, such a knife is worn on a belt or under a pant leg. If worn around the neck (by means of a chain or lanyard) they become a neck knife. Boot knives generally come with a sheath that includes some form of a clip. Most have double-edged blades, like a dagger, that range from 3 to 5 inches (7.62 to 12.7 cm).