Many swords come with a sheath, to store and protect the blade while it is not being used.
Some of these are not tied at the belt, but are carried by the user in their off-hand. This allows it to be wielded in tandem with the sword, and it provides an effective defensive measure.
A sheath may be wielded as a one-handed simple melee weapon. It deals 1d4 bludgeoning damage. It also provides an additional +1 to AC while held.