Comment by ignormies

> This new sense appears to stem from a mistaken belief that the first three letters of nonplus are there to indicate that someone is something other than “plussed” (although what being plussed would entail here remains a mystery)

This is what's called a "lost positive". Rob Words on YouTube has a pretty good video on this topic:

https://www.youtube.com/watch?v=a7TfjCIbtng