"Dunce" comes from the name of the philosopher Duns Scotus.
Well, except that Einstein was an actual person, and Nimrod was a mythological one.
Hardly an important distinction for these intents and purposes; particularly since those stories were widely taught as fact until fairly recently.
Bugs Bunny taunted Elmer Fudd, a stupid an incompetent hunter that failed to catch Bugs Bunny, by calling him a Nimrod. But the children who watched Loony Tunes didn't get the joke and so they assumed it was a synonym for "stupid and incompetent."
Imagine that in the future people think that that "Einstein" means "imbecile" because "you're a regular Einstein" was an ironic taunt. It's the same idea.