"castle in the (air|sky)"의 정의 및 의미

What is the origin of the idiom "castle in the air" and when to use it?

The phrase "castle in the air" is an idiom that refers to a daydream or a fanciful and impractical idea or plan that is unlikely to be realized. The expression likely originated from the idea that castles, traditionally built on solid ground, cannot exist in the air or the sky, emphasizing the unrealistic nature of the described situation or aspiration.