I knew it! Context clues typically imply the ("wrong") meaning, so I misinterpreted this word for a while until I used it out loud, and my pedantic family corrected me.

But so many usages of the word are vague, and the difference between "perplexed" and "ambivalent" is not super important in most stories. (Whoever is nonplussed is not likely to take action on the situation.)

I had a feeling that some authors were using it "wrong." The article does a good job of finding examples where the author's intent was clear. But it's nigh impossible to figure out the intent in most cases. It's fascinating how so many different interpretations can thrive, while not being important enough to correct in most cases.

In my head, "nonplussed" is equivalent to a blank stare. The described person is either not paying attention, or the situation went completely over their head.