Trump is deeply insecure. His insecurity is probably founded on psychological abuse by his parents. I suspect his father often called Donald dumb, or not smart. Trump projects when he feels threatened or in this case when he is reminded of those insults which he knows are accurate. This insecurity, this primal fear, was exposed in his reflexive response demanding that the word never be used around him.
This is important because the cornerstone of almost any competition, especially verbal confrontation, is to knock your opponent off their stride. A strategic advantage is achieved when you force your opponent into an emotional state impairing their ability to respond to your positions or explain their own. Trump of all people knows this and is quite good at it. When Biden accused Trump of not being smart he bested Trump at his own game.Daily Kos Trump’s Tell