Definition: bewildered, unsure how to respond or act OR unexcited; not dismayed
There are two definitions for this, one more recent than the other, and seemingly at odds with each other. To be “nonplussed” means you might be:
Originally, “nonplussed” was Latin for “no more” or “no further,” meaning the person was so confused there was nothing else that could be done. The usage occasionally changes because “plussed” can be be interpreted as “excited,” so to be “nonplussed” is to be not excited.
The former usage is much more accepted than the later. Scholars consider the latter usage simply wrong.
Why I Looked It Up
I found an example of the second usage in a book about Middle Eastern politics:
Kissinger was writhing with embarrassment and anger as the President made the gratuitous remark, but Saqqf was nonplussed. “We are all Semites together,” Saqqaf deftly replied.
Clearly, the author meant that Saqqaf disregarded the remark and was unexcited and unphased by it.
I ran across the word again in a New York Times article about “cancel culture.”
In a sharp essay in Liberal Currents, Adam Gurri looked at empirical evidence that might tell us how big a crisis academic cancellations really are, and he came away nonplussed.
Again, the usage is confusing. I read the essay referred to in the first sentence, and the author could either be confused or dismissive. I’m not sure which.