Things that somebody pretends to despise or puts down because he/she cannot obtain or have.

Example

"I think his comments about that new car are just sour grapes because he can't afford it."

Etymology

From Aesop's fable The Fox and the Grapes, in which a fox, unable to reach grapes it is seeking, convinces itself that they must have been unripe (therefore, sour) all along and so not worthwhile trying for in the first place.

Source

• Wiktionary