out of blue

"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.

Even we never talk to each other before, she decides to see me out of blue.

by Imsohangri March 09, 2018