Hidden Weapon Mechanism (5e Equipment)
From D&D Wiki
Cost: 2 gp
Weight: 0.05 lb.
Either consisting of a spring-loaded wire or a more complex and reliable clockwork device mounted to the user's upper forearm, hidden weapon mechanisms allow a creature to affix one light weapon to their arm as a bonus action. While a creature with such an affixed weapon doesn't have an item in their hand, they may un-affix the weapon, drawing it without using any form of action even if they have used their free item interaction on this round.