What is the origin of the idiom "sour grapes" and when to use it?

The idiom "sour grapes" comes from a fable named "The Fox and the Grapes." In the story, a fox couldn't reach some grapes and, feeling disappointed, claimed that the grapes were probably sour anyway. This phrase is used to describe someone who pretends to dislike or devalue something they desire but cannot have, often out of envy or frustration.