"castle in the air : (idiomatic) A desire, idea, or plan that is unlikely to ever be realized; a..."

“castle in the air : (idiomatic) A desire, idea, or plan that is unlikely to ever be realized; a visionary project or scheme; a daydream, an idle fancy, a near impossibility.”

- https://en.wiktionary.org/wiki/castle_in_the_air#English

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