The etymology of the word doughnut is this: You'd wonder why they were called dough-nuts when they are definitely made of dough, but aren't generally made with nuts. It has nothing to do with nuts; doughnut is a simplification of doughnoughts. They are made of dough and shaped in a ring like a nought. Makes sense when you think about it, doesn't it? As for donuts, well, I imagine that's just another americanism for someone who is too lazy for a couple extra letters.