My favorite example: Dunce. From Wikipedia: "Duns Scotus wrote treatises on theology, grammar, logic and metaphysics, which were widely influential throughout

My favorite example: Dunce. From Wikipedia: "Duns Scotus wrote treatises on theology, grammar, logic and metaphysics, which were widely influential throughout Western Europe... The followers of Duns Scotus were called the Dunses, Dunsmen, or Scotists. When in the sixteenth century the Scotists argued against Renaissance humanism, the term duns or dunce became, in the mouths of the Protestants, a term of abuse and a synonym for one incapable of scholarship." This is similar to how people call others "Einstein" when they do something stupid.