What do you think that word means? Here are your choices:
1. so surprised and confused that one is unsure how to react
2. not surprised or confused
The standard ("correct") definition is #1, and I knew that, but in 2013 I started to think maybe I was wrong because I kept seeing it used in accordance with definition #2.
So I double-checked the definition and saw that #2 is now listed as "nonstandard, informal North American English."
Still, I was really bothered, not because people were using it "incorrectly" - descriptivism compels me to admit that when people start using a word in a certain way, even if that way is prescriptively incorrect, it becomes correct - but because there was no need for this awful word! Every time I saw it, it confused me. You might say, heh heh, that I was nonplussed by the word nonplussed. Why do we need this word? Why are we using it in our published, edited books and articles? Now that it means both its actual definition, as well as that definition's exact opposite, it has become useless. When I see it in your book, or your article, I no longer know what meaning you are trying to convey.
Do you mean the person was shocked? Or do you mean they maintained a completely deadpan mien in the face of some horrible news? It's anybody's guess at this point.
And heck yeah, I went to the corpus (COCA) to provide you with some examples. Here's an example from Good Housekeeping, where it's used
"When I showed them Michael Jackson's Thriller video, my jaded children were nonplussed."
Allow your brain to recover from the annoying double-take it had to do when it came across "nonplussed." Then realize that in this context, the writer means that they were completely unimpressed. The corpus is full of examples like these.
But more troubling are examples like these, where I have no idea what the word is supposed to mean:
"Most would consider The Beatles as relevant today as ever, but there were more than a few contestants who seem nonplussed by the choice."
"Amir paused. He looked quite nonplussed, which, despite the exacerbating circumstances, pleased me inordinately."
"The woman appeared nonplussed by Aliss's outburst."
Were these people shocked, or unimpressed? We will never know.
Nonplussed. I do not think it means what you think it means.