That's a bouncer. A gross exaggeration, a braggart's lie. (Dutch, bonz, verb bonzen, to bounce or thump. A bouncing lie is a thumping lie, and a bouncer is a thumper.)
“He speaks plain cannon, fire, and smoke, and bounce.” —Shakespeare: King John, ii. 2.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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