In Norse mythology, Gleipnir (Old Norse "open one") is the binding that holds the mighty wolf Fenrisulfr (as attested in chapter 34 of the Prose Edda book Gylfaginning). The Gods had attempted to bind Fenrir twice before with huge chains of metal, but Fenrir was able to break free both times. Therefore, they commissioned the dwarves to forge a chain that was impossible to break. To create a chain to achieve the impossible, the dwarves fashioned the chain out of six impossible things:
- The sound of a cat's footfall
- The beard of a woman
- The roots of a mountain
- The sinews of a bear
- The breath of a fish
- The spittle of a bird
Therefore, even though Gleipnir is as thin as a silken ribbon, it is stronger than any iron chain. It was forged by the dwarves in their underground realm of Svartálfaheim.
Gleipnir, having bound the Fenrisúlfur securely, was the cause of Týr's lost hand, for the Fenrisulfr bit it off when he was not freed. Gleipnir is said to hold until Ragnarök, when it will break and Fenrir will devour Odin.