E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Fenrir or Fenris.
The wolf of sin [i.e. of Loki], meaning the goading of a guilty conscience. The wolf was the brother of Hel (q.v.). When he gapes, one jaw touches earth and the other heaven. In the Ragnarok he swallows the sun and conquers Odin; but being conquered by Vidar, he was cast into Nifiheim, where Loki was confined.