the.com/castle in the air

A dream with no foundation, no deed, and no zoning permit.

means An impractical hope or plan built on fantasy rather than anything real or achievable.

from The phrase (and its twin, 'castle in Spain') dates back centuries in English and French, imagining a grand fortress hovering with nothing underneath it — literally a building standing on air instead of ground. 'Château en Espagne' appears in French from the medieval period, possibly referencing the idea of owning fanciful, unearned estates in a foreign land. English writers were using 'castles in the air' for idle, groundless schemes by at least the 1500s-1600s, and it stuck simply because the image does the work: nothing is more obviously unsupported than a castle floating in empty sky.

origin
18th-century phrase for ungrounded hopes or fantasies
cousin term
pie in the sky, same wishful altitude
thoreau's take
said put foundations under them, not down them
real risk
planning life around things that never get built
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