The name Rerir is not explained, and does not appear to have any obvious Elvish derivation. It is perhaps derived from the name of the character Rerir in the Völsunga saga. If so it this is a unique etymology among the place-names in the Silmarillion, though the use of sources like this was not unknown to Tolkien. Many of the names of Dwarves in the Third Age, as well as the name 'Gandalf', come from the Poetic Edda, the same source as the Völsunga saga.
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