<Tr> <Td> </Td> <Td> Look up out of the blue in Wiktionary, the free dictionary . </Td> </Tr> <P> "Out of the blue" is an informal English language idiom that describes an event that occurs unexpectedly, without any warning or preparation . It is used as an adverb . The "blue" in the phrase refers to the sky, one from which a sudden thunderstorm is unexpected . </P> <P> Longer and more explicit variations include out of the clear blue sky and out of a clear blue sky in addition to "out of the duck". </P>

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