take the bit between (one's) teeth

To begin to do something with decisive, stubborn resolve; to act in an bullheaded and unruly manner. A reference to a horse that is no longer under the rider's control because the bit (part of the apparatus used to guide it) is out of place in its mouth. The CEO started ignoring the board's demands and took the bit between her teeth, running the company the way she saw fit. In spite of my counsel, my clients took the bit in their teeth and went public with their accusations.