What does Opinions are like Belly buttons.
Everyone has one. mean?

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@mistletoe68 Example. You and a friend have a brief conversation with a third person. That person expresses an opinion you don't agree with. You don't debate them and you and your friend leave. You want to express to your friend that you disagree with the other persons opinion, but in a humorous way. This would work well.
You could have said it to the third person, but it would be insulting and your intent would be confrontational. There's another stronger version of this, but it's rarely acceptable in a mixed group of people. This is the safe version.