In the Three Kingdoms Period (220-280), the king of the Wei, Cao Rui, wanted to select a very capable man to work for him. He said to his ministers: 'When choosing a talented person, always beware1 of(小心,谨防) one with a false reputation. A false reputation is just like a picture of a cake; it can' t relieve(解除,减轻) hunger.'
Later, this idiom came to be used to mean comforting oneself with unrealistic(不切实际的) thoughts, without solving practical problems.