Meaning of “flop” - Learner’s Dictionary

flop

verb [ I ] uk us /flɒp/ (present participle flopping, past tense and past participle flopped)
flop down/into/onto, etc

to fall or sit somewhere suddenly in a heavy or relaxed way:

He flopped down on the sofa.
HANG

to hang loosely:

Her hair kept flopping in her eyes.
NOT SUCCESSFUL informal

If a film, product, plan, etc flops, it is not successful.

(Definition of “flop verb” from the Cambridge Learner’s Dictionary © Cambridge University Press)