A windup is the backward motion someone makes before throwing something forward with force. When a baseball pitcher winds up, they step back, rotate their body, and swing their arm behind them before releasing the ball toward home plate. That whole preparatory motion is the windup. A quarterback might use a short windup before launching a long pass, and even someone skipping a stone across water does a little windup to build momentum.

The windup stores energy in your muscles and helps you throw harder and more accurately. Think of pulling back a slingshot: the further you pull, the more power you get. Your body works in a similar way.

The word also describes the act of turning a key or handle to tighten a spring mechanism. Old clocks and music boxes need winding up to work. You wind up a windup toy by turning a knob, and then the stored spring energy makes it move across the floor.

People also use windup to mean the final part of something, like the windup of a story or a show.

People also use wind up (two words) as a verb meaning ending up in a situation, often unexpectedly.