Why is doughnut is called doughnut but it doesn't has any nuts ingredients?

Doughnuts are nut-shaped rings of dough. The 'nut' in 'doughnut' is a shape, not an ingredient. In the US, doughnuts are ring-shaped (think hardware, nuts-and-bolts, not peanuts.) In other parts of the world doughnuts are balls of fried dough (walnut-shaped, still not containing nuts.)