Plural of chatterbox; multiple people who talk continuously or excessively.
Chatterbox combines chatter (rapid repeated sounds/speech) and box (a container), suggesting the mouth is a 'box' producing endless chatter. The plural adds -es for regular English pluralization.
A 'chatterbox' is older slang (dating to the 1700s) that literally imagined the mouth as a box nonstop producing sound—it's the same creative metaphor strategy that gave us 'motormouth' centuries later.
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