What is the meaning and origin of the phrase 'a castle in the sky'?

[Priya Raghavan]

English Language Tests, Intermediate level

ESL/EFL Test #317 "English Slang Idioms (3", question 1

"That is just a castle in the .......... I haven't sold a home for that price since 1983," the real estate agent told the couple.

(a) sky
(b) woods
(c) city
(d) books

English Language Tests, Intermediate level

ESL/EFL Test #317 "English Slang Idioms (3", answer 1

"That is just a castle in the sky. I haven't sold a home for that price since 1983," the real estate agent told the couple.

Correct answer: (a) sky
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What is the meaning and origin of this phrase?

Priya Raghavan

[Ralf]

Hi Priya,

'to build castles in the sky' is impossible, so this idiomatic expression describes a fantastic idea. Another expression is 'to have a pipe dream'.
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