Sif, in Norse mythology, the wife of the thunder god, Thor. Sif was a giantess, goddess of grain and fertility, and one of the Asynjur. She was the mother of Ull, god of archery, skiing, and single combat. Sif was Thor's second wife, and Ull was his stepson. Sif's golden hair, which symbolized her connection to the fertility of the land and the harvest of grain, was often praised. She was known to be very vain about its beauty. According to legend, Loki, the mischievous trickster fire god, cut off Sif's hair while she slept (in one version, he burns it off). Thor was so angry that he forced Loki to replace Sif's hair with a wig, crafted by the dwarfs and made of the finest strands of gold.