Meaning/Usage: Pleasant to imagine but impossible to attain

Explanation: Comes from a song written by Joe Hill. It indicated that you will get heavenly rewards after death. However, people who did not believe in heaven turned this phrase into an idiom to express something pleasant that is unattainable.

"Her desire to be an actress ended up just being a pie in the sky."
"His dreams of becoming a famous singer are just a pie in the sky."
"Their ideas about living in space are just a pie in the sky."

A. "I thought she moved to California to become a model?"
B. "She did, but it didn't work out for her."
A. "Seems like her dreams were just a pie in the sky after all."
B. "That's how it seems."

Other Common Sentences

"That's an impractical wish."
"That's an unrealistic dream."