Green’s Dictionary of Slang

1. (US black) a notably large mouth; thus a person with a large mouth.

1960, 1975

Wentworth & Flexner DAS.

2. (US) a talkative person.

1945

W. Pegler 13 Dec. [synd. col.] In the days when I was fearlessly denouncing old satch [i.e. Fiorello la Guardia].