A person or thing that puts caps on something, or something that serves as a finishing touch or climax.
From cap plus the agent suffix -er, creating 'one who caps.' Over time, it also came to mean something that completes or finishes a sequence.
When someone says 'and as a capper to it all,' they're using an old American expression that comes from the circus and theater world, where the 'capper' was the final, most impressive act!
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