Shackles is a quest item in The Witcher 3: Wild Hunt.
It is found near the big boat hull on Undvik and is supposed to be the "twine" Octo asks for during The Lord of Undvik. However, whether due to a bug or intentionally, they don't really serve a purpose as the game will always act as if they are given to Octo once he receives nails. If found before the quest completes, the item will turn into sellable junk.