“A stranger just came up to me out of the blue and gave me twenty dollars. It was really strange.”
“My girlfriend just broke up with me out of the blue.” (
여자친구가 난데없이 저를 졌어요. )
Sometimes something happens “out of the blue” or someone says something “out of the blue”, this means that it was completely unexpected.
This idiom originates with the sky. The idea is that when weather is normal the sky is blue, but when bad weather happens the sky usually changes color, or clouds form. Then from that dark sky you can get rain, thunder or lightning. Imagine though that the sky was completely clear and blue and then in an instant it started raining. Or that from a clear blue sky there was a bolt of lightning. It would be very unexpected for bad weather to happen “out of the blue” sky.
We wouldn’t use this expression for something that was expected. This sentence wouldn’t work “I knew the guy liked me and he was always flirting with me, and today he asked me out out of the blue.” A situation like that isn’t unexpected.
Also, we wouldn’t use it for something extremely tragic “My grandfather died out of the blue.” Would be too light. Idioms are not formal enough for extremely serious situations. “My goldfish died out of the blue” might be fine, if you weren’t too attached to the pet.