Dictionary.com
Thesaurus.com
Synonyms

castle in the air

American  

noun

  1. a fanciful or impractical notion or hope; daydream.

castle in the air British  

noun

  1. a hope or desire unlikely to be realized; daydream

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Etymology

Origin of castle in the air

First recorded in 1570–80

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.