Out of the blue Informal

Completely unexpectedly, suddenly, and without any warning or visible buildup.

It can introduce good news, bad news, or simply a surprising event — the phrase itself is neutral about whether the surprise is welcome.

The image behind the phrase is a bolt of lightning striking from a clear blue sky, with no storm clouds to warn of it — the most dramatically unexpected kind of weather event imaginable. The etymology dictionary Etymonline traces “the blue” as a noun meaning “the sky” to the 1640s, and records the closely related phrase “a bolt from the blue” as a fixed expression by 1837. That year is notable because it is when Thomas Carlyle published “The French Revolution: A History,” a book often credited with popularizing the figurative use of a lightning bolt from a clear sky to describe a shocking, sudden turn of events; the shorter, more casual “out of the blue” is generally understood to be a later simplification of that longer image, gaining wide currency in the twentieth century as the idiom shed its explicitly meteorological framing and came to describe any abrupt surprise, weather-related or not.

Out of the blue, my old college roommate called me after ten years of silence.

The company announced layoffs completely out of the blue, with no prior warning to staff.

She got a promotion out of the blue, without even applying for it.

A: Why did he suddenly quit? B: No idea, it was totally out of the blue.