Castles in the sky noun phrase
Impossible or unrealistic dreams, hopes, or plans
As your friend, I must say that your plan is castles in the sky.
There's no way we can pull that off. It's just castles in the sky.
To daydream
Said when you believe something will never hapen
Said when someone believes in unrealistic or fanciful ideas that is impossible to happen
Used to refer to an impossible plan or idea
Something that is impossible to happen, to achieve, or to be done.
This phrase first appeared in the 1500s. The phrase appears to have derived from the original "to build castles in Spain." Much of Spain was under Moorish authority, so the concept of constructing a castle there was an impossible dream.
The original phrase was first used in the 13th century in Le Roman de la Rose. In 1365, it was translated into English from the original French.