A dolt; a stupid person. The word is taken from Duns Scotus, the learned schoolman and great supporter of the immaculate conception. His followers were called Dunsers. Tyndal says, when they saw that their hair-splitting divinity was giving way to modern theology, “the old barking curs raged in every pulpit” against the classics and new notions, so that the name indicated an opponent to progress, to learning, and hence a dunce.
He knew what's what, and that's as high As metaphysic wit can fly ... A second Thomas, or at once To name them all, another Dunse.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894