Past tense of flap; waved or swung loosely; moved up and down or side to side with quick motions.
From the verb flap (of uncertain origin, possibly imitative of the sound of flapping) plus the regular past tense -ed suffix.
Flap is probably onomatopoeia—it might literally sound like what it describes, like how 'splat' and 'pop' mimic their sounds. We've used this word for centuries without knowing where it came from!
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