More accurately, "pie in the sky" refers to an overly optimistic plan for the future.
"His idea of turning one small shop into a large retail chain was just pie in the sky".
"She has this pie-in-the-sky idea of becoming a model and getting rich overnight".
"The politicians keep making these promises about balancing the budget, it's all pie in the sky".
For the origin of the phrase: Pie in the sky
This earlier, slightly different meaning is from a song that criticizes religious charities for emphasizing the need to suffer on earth so that we can, perhaps, gain our reward later in heaven.
"Work and pray, live on hay,
You'll get pie in the sky when you die."
not a teacher
Student or Learner