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castle in the air noun

  /ˈkæs(ə)l ɪn ði ˈɛəɹ/ , /ˈkɑːs(ə)l ɪn ðiː ˈɛə/
  • (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. [from mid 16th c.]
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