To hit sharply with the palm of the hand, or the sound/force of such a hit.
From Middle Dutch 'smacken,' likely imitative of the actual sound produced by impact. Possibly related to Old High German 'smacchen.' The imitative origin is fitting given what the word describes.
Onomatopoeia like 'smack' exist in every language because humans' brains are wired to connect sounds with meanings—when you hear 'smack,' your brain literally activates the same areas as if you were hit.
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