In the early 1900s, a radical workers’ union used a song called The Preacher and the Slave to blame the church for suppressing the poor with promises of rewards in heaven.
The song included these lines, and from them, “pie in the sky” took the meaning of a false promise.
You will eat, by and by,
In that glorious land above the sky.
Work and pray, live on hay,
You’ll get pie in the sky when you die, by and by.