Making slapping sounds, or moving in a loose, flappy way that creates noise.
From 'slap' (to hit with a flat hand) with the suffix '-y' meaning 'having the quality of.' Describes something that slaps or makes slapping noises.
Skateboarding has a 'slappy' trick where you pop the nose against the ground to make it hop—the sound is literally where the trick gets its name!
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