Extremely large or impressive in size, quantity, or degree.
From 'whop' (to hit or strike hard), which comes from imitative/onomatopoetic origins. A 'whopping' amount is figuratively something that 'hits' you with its impressiveness through sheer size.
The word 'whopping' originally meant a beating or blow, but when you say 'a whopping 500 pounds,' you're metaphorically saying the number is so big it hits you like a physical blow!
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