This one is debated. Some say that it’s rather simple- they used to add nuts to the dough. Others say that specifically- before the holes- a large nut or many small nuts were placed in the center because the center often didn’t cook properly unless the outer sides were burned- the origin of holes in doughnuts is also likewise debated- but one reason given is the exact same- that the hike was put in to allow the dough to cook better and avoid the soggy uncooked center. Of course- bagels had holes and these are believed to be for ease of display on dowels (sticks) so... the hole may have been copied from bagels.
Bagels don’t have the “uncooked center” problem- but it is also important to note that bagels are usually baked or steamed- a different process than frying which is usually a very fast application of high heat- bagels are boiled and the. Cooked further for a longer period- so they have more chance to cook evenly. Thusly bagels wouldn’t need a hole for proper cooking- but... there are donuts without holes and they don’t have the doughy center issue- which casts some doubts in the theory- BUT anyone who’s made donuts knows that if you cook on too low a heat, or too high for too short- you get doughy centers. So
It may have been that with technique and technology of the time- donuts were hard to make well and it was easier to get consistently “good” donuts with a hole in them. I digress though. To the word doughnut-
The other leading theory on the origin of the word is that they were called “dough knots”- as early examples were “twists” or “knots” and are usually attributed to the Dutch- although the oliekoek is another subject and at least in the present day- not usually made into knots and traditionally were balls- which one could argue could look like a “walnut” or other “nut” made of dough I suppose- although that isn’t a theory I’ve encountered- just one I am putting out there to show there are many possibilities.) So really- no one knows- or maybe one of the theories is right and no one can conclusively prove- the origin of the word.
Looks like people aren't that evolved for greatness.