E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Bit (of a horse).
To take the bit in (or between) his teeth. To be obstinately self-willed; to make up ones mind not to yield. When a horse has a mind to run away, he catches the bit between his teeth, and the driver has no longer control over him.
Mr. X. will not yield. He has taken the bit between his teeth, and is resolved to carry out his original measure.Newspaper paragraph, April, 1886.