Meaning:

Suddenly and unexpectedly

If someone does something or something happens out of the blue, it happens suddenly and surprises you because you didn't expect it.

Example:

Late last summer, out of the blue, Sue announced she was getting married and leaving her home country soon. We were all shocked to hear that.

We had been told it would be sunny all day, so we went hiking. However, it started to pour (rain heavily) out of the blue in the afternoon.

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