When ready to join the Faction, an initiate was taken to the Stacks to choose a weapon. Most chose a bladed weapon, which is how the term came into use. The weapon was causally removed, leaving only the meaning of the object, which was then connected to the initiate. Observers could see it as a silhouette wielded by the Faction member's shadow.
Most Cousins could only carry one shadow weapon, but Faction lore said that Grandfather Paradox's four lieutenants not only had several such weapons each, but were also shadow armoured as well. (PROSE: The Book of the War)