Slinger (5e Equipment)
From D&D Wiki
Martial Ranged Weapons
The slinger is a tool which mounts a slingshot onto the arm so that it can be drawn with the opposite hand. These are by no means common, being made particularly for those of ranger groups or other reconnaissance specialized entities. It can be loaded with things usually placed in a sling.
Proficiency with a slinger allows you to add your proficiency bonus to the attack roll for any attack you make with it.