A hot dog is an item between two pieces of bread
Meat and a bun/bread and a filling fit the two requirements for a sandwich.
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The basic definition of a sandwich in the Merriam-Webster dictionary is "two or more slices of bread or a split roll having a filling in between". That fits the description of a hot dog PERFECTLY: it's a split roll with a piece of meat in the middle. Having the definition in the most commonly used dictionary in the world certainly makes an irrefutable argument in favor of the hot dog's standing as a sandwich. A sandwich is a hot dog, and in the same such way a hot dog is a sandwich.
The dictionary does not specify EXACTLY if a hot dog can be classified as a sandwich. The definition of a hot dog in the same dictionary also does not match the definition for a sandwich. It's like rectangles and squares - a rectangle is a square but a square is not considered a rectangle.