Tending to flap or move loosely and rapidly; having characteristics that cause something to flap or wave about.
From the verb 'flap' plus the adjective-forming suffix '-y.' The '-y' suffix turns actions or nouns into descriptive adjectives, as in 'rainy' from 'rain' or 'sleepy' from 'sleep.'
The '-y' suffix is one of the most productive in English—we unconsciously add it to almost any noun or verb to make an adjective, and native speakers immediately understand new words like 'flappy' even if they've never heard them before.
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