a fable (=a traditional short story that teaches a moral lesson) by Aesop in which a fox tries many times to reach some grapes that are hanging above his head. When he realizes that he will never be able to reach them, he pretends that he never wanted them, saying "They're probably sour (=having an unpleasant acid taste) anyway". This attitude is called sour grapes.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.
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