what's that got to do with the price of cheese in China

The phrase is used to mean “How is this relevant to the conversation?”, especially if a person seems to be actively avoiding the topic being discussed.

Me: Hey, do you have that $20 bucks you owe me.

Him: Hey, have you seen the movie Hot Tub Time Machine yet?.

Me: What's that got to do with the price of cheese in China?

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