castle in the air
Definition from Wiktionary, the free dictionary
- (idiomatic) A visionary project or scheme; a day-dream; an idle fancy; a pipe dream; any plan, desire, or idea that is unlikely to be ever realized; a near impossibility.
- Completing the metaphor, one is said to build a castle in the air.
idea that is unlikely to be ever realized
- Oxford English Dictionary, second edition, 1989