Hitting something hard and loudly; striking with force, often with a stick or hand.
From 'whack' (onomatopoeia, mimicking the sound of a hit), combined with the gerund suffix '-ing.' The term became common in English in the 1800s.
The word 'whack' is purely onomatopoetic—it literally imitates the sound it describes, which is why it sounds satisfying and why video games use the sound of whacking for feedback!
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