Sour grapes
Definition 1 of 1
Pro Tip 1/3Hidden Desire
The phrase implies a hidden wish for something unattainable, masked by disdain. 
Saying she didn't want to go to the prom was just sour grapes because she wasn't asked.
Pro Tip 2/3Criticism Mask
Often used to suggest that criticism is a defence mechanism for envy or disappointment. 
He derided the award-winning novel as overrated, but it felt like sour grapes.
Pro Tip 3/3Aesop's Fable
Originates from Aesop's Fable of a fox who calls grapes he can't reach sour, indicating false disparagement. 
Ignoring her obvious skill, he said she only won by luck – a clear case of sour grapes.