The slender fetter made of impossible things that bound the Fenrir wolf.

Gleipnir was forged by dwarves in Svartalfheimr from six impossible things: the sound of a cat's footsteps, a woman's beard, the roots of a mountain, the sinews of a bear, a fish's breath, and a bird's spittle. Despite its silken appearance it was impossible to break.

The Æsir tricked the Fenrir wolf into allowing itself to be bound with Gleipnir against a promise of release, a promise they never intended to keep. Týr sacrificed his hand as surety for the gods' pledge when the wolf sensed the deceit.

Attestations

Gylfaginning 34
Snorri recounts Gleipnir's creation and the binding of the Fenrir wolf.