To fall or drop heavily. (Cambridge Dictionary)
If a book, play, film, etc. flops it is not successful. (Cambridge Dictionary)
If you flop into a chair, for example, you sit down suddenly and heavily because you are so tired. (Collins Dictionary)
If something flops onto something else, it falls there heavily or untidily. (Collins Dictionary)
To swing or move loosely. Flap. (Merriam – Webster)
To throw or move oneself in a heavy, clumsy, or relaxed manner. (Merriam – Webster)
Fall, move, or hang in a loose and ungainly way. (Oxford Dictionaries)
Why Do People Have “Flop”.