What is the meaning of "out of the blue"?

The meaning of "out of the blue" is:

without warning


What is the definition of "out of the blue"?

The definition of "out of the blue" is:

without warning



"out of the blue" means

without warning

Example Sentences:

I was sitting at home, reading the newspaper, when out of the blue my best friend from high-school called. I haven’t talked to her in 20 years!

Out of the blue, the policeman ran into our office and began to chase the criminal.