Shackle

The Shackle is an Accessory that grants 1 point of Defense. It is dropped by Zombies somewhat rarely, although it's easy to acquire in plentiful amounts if you venture into the wilderness during a Blood Moon and keep killing zombies.

They become obsolete near the end-game, as the player can obtain an Obsidian Skull or Cobalt Shield, which grant immunity against burning blocks or knockback, respectively. (Note: since players may not equip duplicates, wearing a Shackle in combination with the above is now required to achieve maximum defense bonus from accessories.)

They can be sold to a Merchant for 3 Silver Coins.

Trivia

 * In real life, a shackle is actually a metal link used for linking devices together.