bite down

1. To bite very hard and purposefully (on something). I nearly lost a tooth biting down on that apple! I knew there was something wrong with the burger the moment I bit down. As soon as she bit down on her sandwich, her loose tooth came out!

2. To clench one's teeth very firmly. I keep trying to give the dog her pills, but she's biting down so hard that I can't get them into her mouth. The field surgeons didn't have any anesthetic, so the poor man had to just bite down while they operated. The doctor finally had to give Sally a syringe of liquid medicine because she would bite down every time he got close to her with the pills.