A chatterbox is someone who talks too much — especially a child, though adults get the name too. It is informal, and usually warm or only a little annoyed, not a hard insult. Parents say it when a child cannot stop telling stories. The picture is a box so full of chatter that talk keeps spilling out; it is not a real box. In writing it can sound slightly old-fashioned or cute. Teenagers might just say someone never stops talking. Though actually, it comments on how much a person talks, not on whether the talk is clever or true.