Định Nghĩa và Ý Nghĩa của "pie in the sky"

What is the origin of the idiom "pie in the sky" and when to use it?

The idiom "pie in the sky" originated in the early 20th century, and it is often attributed to labor union songs and slogans. It has been used to describe an idea or promise of something wonderful or beneficial that is highly unlikely to materialize, much like a pie that is in the sky is out of reach.