knockout punch

1. In boxing and similar sports, a blow that renders someone unconscious or technically disqualifies them from continuing. The newcomer delivered an incredible knockout punch in the second round, stunning just about everyone. She was behind for nearly the whole match, but she won with an incredible knockout punch that laid her opponent out flat.

2. By extension, any action or maneuver that completely and decisively ensures the defeat or downfall of someone or something. The Supreme Court's decision delivered a knockout punch to our hopes of overturning the legislation. Their company was already struggling to survive, but the arrival of a new big-box retailer next door proved to be the knockout punch.