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.
second-guess somebody/something
1. to try to guess what will happen or what someone will do It's not for us to second-guess the court's decision - we'll just have to wait and see.
2. to criticize someone's actions or an event after it has happened It's easy to second-guess the team's coach - but let's face it, he made big mistakes.