To anticipate or predict someone's actions or thoughts. She tried to second-guess what her boss would say in the meeting.
To criticize or question the decisions or actions of someone, often after the event. It's easy to second-guess the coach's decisions after the game is over.
To doubt or reconsider a decision, judgment, or opinion, especially one's own, often causing hesitation or delay. Don't second-guess yourself—trust your first instinct.