The meaning of "吃葡萄不吐葡萄皮儿."

[Wang7]

"Eat grapes, don't spit out grape skins," definitely a tongue twister, but is it an idiom? If so, what is the context? Thanks.

[Kamille]

It's only a tongue twister. 

There is an idiom involving grapes that looks similar though: 吃不到葡萄說葡萄酸 (meaning that if someone can't reach something they want, they'll tell other people that they actually don't care in order not lose face).

[tooironic]

I believe that idioms derives from the Greek sour grapes.