1 collapse, drop (down), fall (down), tumble, topple, plump down, plop down, flounce down:
I was so exhausted that I flopped into bed at once.
2 flap, wave, swing:
The door, unfastened, flopped back and forth in the wind.
3 fail, fall flat, come to naught or nothing, founder, Colloq fold, US bomb:
The musical flopped on the first night.
4 failure, fiasco, disaster, non-starter, débâcle, US fizzle, Colloq dud, washout, clanger, US lead balloon, bomb; Slang lemon, Brit cock-up, damp squib:
His idea for a new corkscrew was a complete flop.