A talkative person. The Germans have Plaudertasche (chatterbag). Shakespeare speaks of the clack-dish. “His use was to put a ducat in her clack-dish” (Measure for Measure, iii. 2)— i.e. the box or dish used by beggars for collecting alms, which the holder clatters to attract attention. We find also chatter-basket in old writers, referring to the child's rattle.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894