Grapes can be used to make wine. Sour grapes, though, do not sound so tasty.
Sour Grapes 酸葡萄心理
Anna: Did your buddy Craig ask my friend Morgan out for a second date yet?
Jonathan: Well, he did... and she turned him down. But Craig told me he wasn’t that interested in her anyway.
Anna: Sounds like sour grapes to me.
Jonathan: You are probably right.
This phrase comes from the ancient Greek storyteller Aesop. A fox tries to eat some grapes from a vine. But they are too high for the fox to reach. So the fox claims the grapes are probably sour anyway. When someone has a case of “sour grapes”, they criticize something desirable that they know they cannot have. Just like Craig did after Morgan turned him down.