fool me once, shame on you, fool me twice, shame on me

Fool me once, shame on you, fool me twice, shame on me is a long idiom that is referring to being tricked and learning from your mistakes. If you get fooled once, it was the first time and therefore were not expecting it. After being fooled, you should be wary in the future of this person who tricked you. However, if you get fooled a second time in the same manner, it is shame on you because you were not cautious and didnt learn from your mistakes.