in Scandinavian mythology, is the name of the bridge between heaven and earth; the rainbow may be considered to be this bridge, and its various colours are the reflections of its precious stones. (Icelandic, bifa, tremble, and rost, path.)
The keeper of the bridge is Heimdall. It leads to Doomstead, the palace of the Norns or Fates.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894