A full glass, generally connected with a “toast.” Dr. Arn says a bumper is when the surface of the wine bumps up in the middle. (French, bomber, to render convex, to bulge or swell out.)
“A fancied connection with bump, a swelling, has not only influenced the form of the word, but [has] added the notion of fulness.” —Skeat: Etymological Dictionary.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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