We've heard it concerning NATO. We've heard it concerning health care. We've heard it concerning taxes. We've heard it concerning trade with China.
Donald Trump has "flip-flopped" on each of these issues, or so says the conventional wisdom of the Beltway. In a town where almost no problem is ever solved, and no serious issue is ever resolved such that it's no longer an issue, the thought leaders have a particular way of labeling the way politicians manage their issue stances. Say one thing on a given day, modify your stance on a different day, and you "flip-flopped."
In addition to being a very dumb emphemism, this is also an astonishingly stupid way of talking about how policy is made. It's one thing to deride someone like John Kerry, who will sit there and make a statement like, "I actually did vote for the $87 million before I voted against it." That's clearly a case of a politician trying to play both sides of an issue.