The definition of pie in the sky refers to an idea, thought or dream that is extremely unrealistic, even to the point where it begins to seem ludicrous.
A dream that is not likely to ever come true is an example of something that would be described as pie in the sky.
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a promise of benefits or a reward in an afterlife or in the remote future an unrealistically Utopian plan or project Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc. pie in the sky - Cultural Definition
A preposterously optimistic goal: “The candidate says we can balance the budget by next year, but I think that's pie in the sky.”
The New Dictionary of Cultural Literacy Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved. pie in the sky - Phrases/Idioms
An empty wish or promise, as in
His dream of being hired as a sports editor proved to be pie in the sky. This expression was first recorded in 1911 in a rallying song of a union, the International Workers of the World (or “Wobblies”): “Work and pray, live on hay, you'll get pie in the sky when you die.” The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.