Acting meanly after a disappointment.

In the fable The Fox and the Grapes, which is attributed to the ancient Greek writer Aesop, the fox isn't able to reach the grapes and declares them to be sour - "the grapes are probably sour anyway!".

Some of the fables associated with Aesop were written as late as 1900. It isn't clear when the Fox and the Grapes story originated.

The phrase also occurs in the Bible, Ezekiel XVIII 2 (King James Version):

The fathers have eaten sour grapes, and the children's teeth are set on edge.