Since horses’ teeth and gums wear down over time, one can tell a horse’s age and quality by examining its mouth.  Therefore…

• To “look a gift horse in the mouth” is to scrutinize the quality of a gift.

• Since old horses have receded gums and overexposed or “long” teeth, to seem “long in the tooth” is to appear old or worn out.

• In the early days of horse racing, racetrack tipsters elicited bids by claiming the horse itself had promised to win.  The guarantee came “straight from the horse’s mouth.”

Before the invention of anesthetic, soldiers undergoing surgery literally “bit the bullet” to dull their screams and distract them from the pain.

Jamaican soldiers in the early 1600s had only one-shot weapons.  To compensate for the limited number of shots, they carried armfuls of weapons and extra shot in their teeth.  They were “armed to the teeth” and ready for battle.

At the turn of the century, a man with long sideburns, or “mutton chops,” was “busted in the chops” if he was hit in the face during a brawl.  Nowadays, the term indicates a figurative in-good-humor brawl:  “don’t be offended, I’m just busting your chops.”

If you are sad and “down in the mouth,” the corners of your mouth are turned down in a frown.

One way to suppress laughter is to bite your tongue with your side teeth.  To say something “tongue in cheek” is to say something funny in a mock-serious tone.

In the British Empire, British school boys were trained to contain their emotions by keeping a “stiff upper lip” in preparation for being soldiers.

The expression “tooth and nail” originated in sixteenth-century British literature that challenged readers to cling to the truth as if with their teeth and fingernails.