A muzzle is a guard that covers an animal's mouth to keep it from biting. If your dog is aggressive, you might have to put a muzzle on her when you take her to the dog park.
Muzzle comes from the Latin morsus meaning "bite." A muzzle is the nose and mouth of an animal like a cat or dog. It's also the name for a guard made of straps or wire that's worn over this part of an animal's face to keep it from biting. It's the mouth of a gun, too. If you muzzle someone, you gag him to prevent him from speaking.