To flop is to drop or hang heavily and loosely. If you're exhausted at the end of the day, you might flop into a chair as soon as you walk into your house.
Your long hair might flop into your eyes, and if you catch a fish, you'll have to watch it flop around in your boat before it stops breathing. A tired child might begin to sob and flop onto the floor in frustration. You can also call a disaster a flop: "We worked so hard on that play, but it turned out to be a total flop." The earliest meaning of flop was "to flap."