noun
a sudden and unexpected event or situation
adverb
unexpectedly or suddenly
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The CEO's resignation announcement came out of the blue, surprising everyone in the company.
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I haven't seen Sarah in years, but she called me out of the blue yesterday.
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I was just chilling at home when my friend showed up out of the blue.
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The idea for the new product design came to me out of the blue while I was taking a walk.