E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Down in the mouth. (See under DOWN.)
His mouth was made, he was trained or reduced to obedience, like a horse trained to the bit.
At first, of course, the fireworker showed fight . but in the end his mouth was made, his paces formed, and he became a very serviceable and willing animal.Le Fanu: House in the Churchyard, ch. xcix.