Bit (of a horse)

To take the bit in (or between) his teeth. To be obstinately self-willed; to make up one’s 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.