Fist Weapons are a type of fast-attack one-handed class-specific weapon for the Monk in Diablo III. They are very similar to (and, to some point, reincorporate) the Katars from Diablo II. Each Fist Weapon is a blunt or bladed piece attached directly to the Monk's arm, via a short fist-grip handle or a strap. Knuckles, spikes, bladed fists and katar swords are all classified as Fist Weapons.
"Fist weapons allow monks to utilize their well-practiced hand-to-hand combat styles, but supplement them with knife blades, toughening hand wraps, or knuckle spikes. "— Game Guide(src)
Fist Weapons may not be transmogrified into any other weapon types.