out of the blue
- 1Suddenly and without any warning or preparation; happening when no one expects it.
Like a phone call from someone you haven't heard from in years, arriving on an ordinary afternoon.
His resignation came out of the blue, stunning the entire department.
Out of the blue, an old college friend showed up at my front door.
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Origin
This is a shortened form of "out of the blue sky," likening an unexpected event to lightning or rain that arrives suddenly from a cloudless sky.