Something exceptionally large, or a big lie or false story.
From 'whop' (to hit or strike hard, from Scottish), combined with '-er' suffix. Developed in American English in the 1800s meaning something huge, or a story 'big' in the sense of being false.
The most famous 'whopper' might be Abe Lincoln's log cabin—historians found no evidence he was born there, but this 'big lie' became an American myth and actually helped him get elected!
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