Why do they call it "hamburger" when it's made of beef?
The common belief is that the American hamburger borrowed its name from a dish called "Hamburg Style Beef" or "Hamburg Steak" which arrived in the United States from the German city of Hamburg in the 19th century. The dish was nothing more than chopped meat eaten raw.
The ground-beef hamburger and bun sandwich combination that we are all familiar with today first appeared in America at the 1904 World's Fair in St. Louis.
—The Fact Monster