The Reinforced Knife is an unlockable gadget upgrade that allows you to cut through rope barriers. This item is optional gear that you do not need to complete the main story. However, if you want to access every area and complete all puzzles, you will need it. You can buy it from a merchant in The Hidden City (Paititi). The vendor is found in the market square (north of the bridge to the Upper City). The item to buy from her is called "Knife Upgrade" and costs 2,900 gold. Alternately, find an NPC on some stairs in the Upper City to begin the "Stay Of Execution" side quest. Successfully complete the quest to unlock the Reinforced Knife upgrade.