Wiktionary
n. A knife that is carried inside a sheath in or attached to a boot. Usually having a fixed double edged blade.
Wikipedia
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. 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).
Usage examples of "boot knife".
Mudge's own weapons lay scattered on the floor nearby, even to his secret heel-boot knife.
She fished in her belt pouch, pulled out the bit of wood she'd cadged off Lipitero and began working on it with her boot knife.
Riyan considered struggling, abandoned the idea as too noisy, and was just about to go for his boot knife when he realized the hand over his mouth wore rings.
Miles pulled his boot knife and used the point to disengage and extinguish the tiny transmit-indicator light in his own wrist comm, then blew into it.