Sour Grapes

This comes straight from one of Aesop's fables, traditionally credited to a Greek storyteller who may have lived around the 6th century BC. A fox spots a bunch of grapes hanging just out of reach on a high vine, tries repeatedly to jump up and grab them, fails every time, and finally struts off declaring that the grapes were probably sour and not worth eating anyway. The phrase entered English through William Caxton's 1484 translation of Aesop's fables, and the psychological trick it describes, devaluing something the moment you realise you can't have it, was so recognisable that the phrase has stuck for over five centuries.