Benz is an informal name for a Mercedes-Benz — the German car with the three-pointed star. People say “a Benz” the way they say “a BMW”: they mean the brand, not one special model. In a shop or a textbook you will see the long name Mercedes-Benz; in the street, Benz is enough. In Britain some people say a Merc instead; in American talk, Benz is the short form you hear. It is casual, not rude — just shorter. Though actually Benz was first a person’s name (a German family name). Karl Benz built some of the first cars; the Mercedes part was added later. So when someone says Benz today, they almost always mean the car, not the man.