1) Something politicians or political groups do. They agree with something then later disagree with it, then later agree with it again an unspecified amount of times.
2) Something members of the opposite sex do. They flirt with you then ignore you, as if they are flip-flopping about whether they like you or not.
1) The Greens are often flip-flopping on many issues, eg illegal drugs. They said they would legalise them, now they say they will not.
2) "Man, that guy is a flip-flopper of the highest degree. He asked me out on Friday and on Monday he told me he was getting back with his ex."