clincher

/ˈklɪntʃər/
noun
  1. A fact, argument, or event that finally decides or settles something.
    • Her final point was the clincher that won the debate.
    • The team's last goal was the clincher in the championship game.
    • The clincher was the email proving he was lying.
  2. A tool or device used to fasten or secure something tightly.
    • The mechanic grabbed a clincher to tighten the bolt.
    • A clincher is essential for securing the rope on a boat.
    • He used a clincher to bend the nail over.