Nowadays when somebody expresses sour grapes, it means that they put down something simply because they can't have it.
The phrase is often used incorrectly as another way to express bitterness or resentment.
the story is that a fox sees some grapes high up0n a tree, he tries to get them,but after many failed attempts,the fox walks aways and says "they are probably sour anyway"
when somebody says "thats sour grapes" they are talkign about somebody desiring something,but unable to have it,so they pretend like they dont want it.when infact they do!
guy2: dude she h8s blonde guys!
guy1: oh i dont care she is probably a dyke anyway
Making several attempts to get a girl you want, getting rejected by that girl, then you realizing somewhere down the line that you dont want her because shes probably sour