Gleipnir is the binding that holds the mighty wolf Fenrisulfr, a monstrous wolf. It is composed 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.