A hotdog is a "frankfurter," a hunk of meat in a bun. Get one at the hotdog stand at a baseball game and slather it with ketchup and mustard for an all-American meal.
The name "frankfurter" illustrates the hotdog’s German heritage — the original came from Frankfurt, Germany, based on a similar pork sausage. There was a suspicion in the 1800s that they were made from dog meat, which is where hotdog came from. If someone calls you a hotdog, they mean you're a daredevil or a risk-taker. Also, you can say “hotdog!” to express delight, kind of like “woohoo!” Hotdog!