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1. To question or doubt someone or something retroactively. Why are you second-guessing me now? You agreed with this course of treatment just yesterday! You made the best decisions you could with the information you had at the time, so please don't second-guess yourself.
2. To try to anticipate how something will happen or what someone will do. Trying to second-guess the outcome of a game in progress is exhausting.
to try to predict what some person will do before it is known to anyone, including the person. There is no point in trying to second-guess Bob. He is completely unpredictable.