Definition of "dunce"

When a true genius appears in the world you may know him by this sign; that the dunces are all in confederacy against him.

c. 1713, Jonathan Swift, “Thoughts on Various Subjects”, in The Works of the Rev. Jonathan Swift, volume 5

[...] Dunce, / Dotard, a-dozing at the very nonce, / After a life spent training for the sight!

1855, Robert Browning, Childe Roland to the Dark Tower Came, section XXX