A bellyflop is a jump into water where the front of your body hits the surface flat, instead of cutting in with the hands and head first. The water slaps your belly; it stings, and people often laugh. Children hear it at the pool. You can also use it as a verb: he bellyflopped off the side. Though actually, people stretch it: if a film or a product does a bellyflop, it fails in a loud, flat way — same picture, no water. That looser use is informal. The swimming sense is the home meaning, and it is a bit comic; nobody is proud of a bellyflop unless they did it on purpose to make friends shout.