What's the origin of the proverbial phrase, "Sky falls/does not fall big pancakes"?
天上掉馅饼，means that free and delicious food like pies are falling from the sky. Or its negative form: 天上不会掉馅饼

Nowadays it usually means that people who are lazy are expecting something miracle but impossible will happen to them. For example:


  不好好工作就想升职，别做梦了，天上又不会掉馅饼。


This phrase came from a Chinese myth. After 嫦娥(the lady in the legend who swallowed the elixir stolen from her husband and flew to the moon) went to the moon, 后羿(the man in the legend who shot 9 suns out of 10 and saved the earth from the heat) was very sad and thought about her every day. When 嫦娥 found out his love, she wrote her love on a pie. Because she is on the moon, there came the phrase: 天上掉馅饼