A frankfurter is a long, thin sausage, usually reddish, that has been smoked. People often put it in a long soft bun and eat it as a hot dog, with mustard or ketchup. The name comes from Frankfurt in Germany — it first meant “from Frankfurt,” and then it meant the sausage that city was known for (the city is Frankfurt-am-Main). In the US, everyday talk often says frank or hot dog instead; frankfurter sounds a little more like a packet or a menu. It names the sausage itself, not a person from that city, though that was the older meaning of the German word. It is an ordinary food word: not formal, not slang.