Green’s Dictionary of Slang

1. a pulpit.

1714

A. Smith Lives of Most Noted Highway-men, etc. I 274: The Fanatical parson being ashamed of himself, he quitted his Chattering Box without proceeding any farther in his Sermon.

2. (UK prizefighting) the mouth.

1847

Era (London) 18 July 6/2: The latter countered [...] and got in on the chattering-box with his right.