To hit or strike something hard with a sharp blow; can also mean to kill or to share something by dividing it into parts.
Likely imitative in origin, coming from the sound 'whack' itself. First recorded in English around the 1600s as onomatopoeia—a word that sounds like what it means. It became slang for killing (whacking someone) in 20th-century crime fiction.
The word 'whack' is pure onomatopoeia—it literally sounds like the action it describes, which is why it's been used the same way for centuries and why everyone immediately knows what you mean without explanation.
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