A flap is a piece of material attached on one side that can move up and down or back and forth. The flap on an envelope folds over to seal it shut. A dog door has a flap that swings both ways so pets can go in and out. Airplane wings have flaps that pilots extend during takeoff and landing to change how air flows over the wing.

The word also describes the motion itself. When a bird flaps its wings, it moves them up and down to fly or hover. A flag flaps in the wind, snapping and waving as the breeze catches it. If you're hot, you might flap your shirt to get some air moving.

When people say they're in a flap, they mean they're flustered or upset about something, like when you suddenly realize you forgot your homework. The image is of someone waving their arms around in panic, like a bird flapping wildly. People also use flap to mean unnecessary fuss or commotion: “Don't get into a flap about it” means stay calm, the situation isn't as serious as it seems.