to make a dunce of; to make dull, stupid, slow-witted or confused ...1594
from dunce (n. a person who is slow at learning or of dull intelligence) + -fy
FIRST DOCUMENTED USE
1594 - see EXAMPLE below
"...This tale must at one time or other giue vp the ghost, and as good now as stay longer, I would gladly rid my hands of it cleanly if I could tell how, for what with talking of coblers, & tinkers, & rapemakers, and botchers, and durtdaubers, the marke is cleane gone out of my muses mouth, and I am as it were more than dunsified twixt diuinitie and poetrie. What is there more as touching this tragedie that you would be resolued of? saie quickly, for now my pen is got vpon his féet again..."
From: The Vnfortunate Traueller. Or, The Life of Iacke Wilton
- Thomas Nashe