Moving with a loose, heavy, or clumsy motion; also means failing badly or completely not succeeding.
From 'flop,' likely imitative in origin (meant to sound like the noise of something falling or moving loosely). Used since the 1600s, with the meaning of 'failure' added in the 1900s.
The word 'flop' is onomatopoeia—it literally sounds like what it describes—and when movies or products 'flop,' they're failing with the same kind of heavy, ungraceful motion as a fish flopping on deck.
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