I'm surprised that no one defined the actual term itself in this node: A chatterbox is a person who just can't shut up: someone who, for no good reason at all, continues to talk and talk and talk until every avenue of discussion has been travelled.
We all hate them. I used to turn into a chatterbox while under the influence. More specifically, while under the influence of alcohol, or pot.
Chances are fairly decent that a chatterbox is usually a backseat driver, or some other sort of layabout or ne'er-do-well.