Sif is the golden-haired wife of Thor, a goddess linked to the earth, harvest and family, whose famous hair, widely read as an image of the ripe grain of the fields, is the hinge of one of the liveliest tales in the Prose Edda. Loki cuts off her hair as a malicious prank, and Thor's fury forces him to make amends: Loki goes to the dwarves, the master-smiths, who forge not only new hair of living gold that grows like real hair, but, in the same wager, the greatest treasures of the gods, Odin's spear Gungnir, Freyr's ship Skidbladnir, and Thor's own hammer Mjollnir. So Sif's stolen hair is, in the logic of the myth, the reason the gods possess their defining weapons.
Beyond this story little is told of her, and her older significance, perhaps as a grain- or earth-goddess matched to the thunder-god of the fertile storm, is a matter of inference. The tale is preserved specifically in chapter thirty-five of Snorri's Skaldskaparmal, the section of the Prose Edda devoted to poetic diction, since the episode was prized above all as the origin-story behind a cluster of kennings, gold called "Sif's hair," Thor called "Sif's husband"; the earlier poem Hymiskvida uses the kenning without ever telling the story behind it, so the fullest narrative comes from Snorri's thirteenth-century prose alone, a reminder of how much of the liveliest Norse mythology survives only because a medieval scholar troubled to explain the poets' own riddling vocabulary.