kicker
The surprising final detail that suddenly changes a story.
The kicker is the surprising twist or unexpected detail that changes everything. When your friend tells you a story about finding twenty dollars, then adds “and it turned out to be counterfeit,” that second part is the kicker. It's the punch line, the gotcha, the part that makes you say “wait, what?”
In football, a kicker is the player who kicks field goals and extra points. They're specialists who might stand on the sidelines most of the game, then jog onto the field when points are needed. A strong kicker can win games in the final seconds.
The word can also mean a hidden problem or catch. If someone offers you a great deal on a used bicycle, you might ask “what's the kicker?” Maybe the brakes don't work, or the previous owner raced it through mud puddles. The kicker is that one detail that wasn't mentioned up front, the thing that completely changes the situation.
Writers and comedians rely on kickers. The best stories save something surprising for the end. You think you know where things are headed, then the kicker flips your expectations. That moment of surprise, when you realize there was more to the story, is what makes the kicker so powerful.