Moving something quickly up and down or side to side (like a bird's wings), or the sound and motion produced by this action.
From Middle English 'flappe,' likely imitative in origin (mimicking the sound). The '-ing' form shows the continuous action of flapping.
Many 'fl-' words in English are imitative—'flutter,' 'flip,' 'flop,' 'flap'—and linguists think this might be because our brains naturally associate the 'fl' sound with quick, bouncy movements, making the sound itself almost pictorial.
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