Great Odin's Raven!

Definitions

(Expression) An expression of great surprise, rarely used. First used in the Anchorman films.

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(Expression) An expression of great surprise, rarely used. Odin's two ravens Huginn and Muninn are in north mythology connected to the concepts of thought (Huginn) and memory (Muninn).
Therefore the expressions ''Great Odin's Raven'' is another way of saying ''I can't recall seeing/hearing/experiencing this'' or ''I never imagine seeing/hearing/experiencing this''