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.

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 as well as granting the defense bonus. Wearing a Shackle in combination with the above is required to achieve maximum defense bonus from accessories.

They can be sold to a Merchant for.

Trivia

 * In real life, a shackle is either a metal link used for linking devices together or a device used to chain people into prisons.