Sour grapes is the action of making something seem less important after finding out they can't have it.
An example of sour grapes is a man saying he didn't want to date a certain woman because she was dumb, after she decided to date someone else.
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a scorning or belittling of something only because it cannot be had or done
from Aesop's fable in which the fox, after futile efforts to reach some grapes, scorns them as being sour
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
Denial of the desirability of something after one has found out that it cannot be reached or acquired:
The losers' scorn for the award is pure sour grapes. The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved. sour grapes - Phrases/Idioms
Disparaging what one cannot obtain, as in
The losers' scorn for the award is pure sour grapes. This expression alludes to the Greek writer Aesop's famous fable about a fox that cannot reach some grapes on a high vine and announces that they are sour. In English the fable was first recorded in William Caxton's 1484 translation, “The fox said these raisins be sour.” The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.