To be nonplussed is to be (1) at a loss of what to think, or (2) bewildered. These are the traditional definitions, anyway. The word comes from the old but now little-used noun nonplus, which refers to a state in which nothing more can be said or done, so to be nonplussed is essentially to be at a standstill or an impasse.
Regardless of the traditional definitions, the word is very often used as a synonym of unimpressed, indifferent, or unfazed—for example:
I was nonplussed by the curry chicken, which I found lacked the flair of Thai or Indian curries. [Bend Bulletin]
Back in London and once again nonplussed by a setback, Seacole asked friends in the military to help her raise money to pay her debts. [A to Z of Women in World History, Erika A. Kuhlman]
Tea Party activists—many of whom are nonplussed about the prospect of a shutdown—plan to rally outside the Capitol on Thursday. [Time]
The use of nonplussed in its traditional sense is increasingly hard to find in popular sources, suggesting the old sense of the word may be fading from the language. Still, it’s possible to scrape up a few instances such as these:
Some of the witnesses are nonplussed. “I don’t understand what it’s all about,” said Irwin Hersh. [New York Times]
He tried to explain his feelings to his friends and family, but his passion left them nonplussed. [The Looming Tower, Lawrence Wright]
According to Gråbøl, the Danes were initially nonplussed by the crepuscular light, as many viewers struggled with their TV brightness settings. [Guardian]
Nonplussed is one of those troubling words that seems to come up most often in discussions of its use and misuse. People do still use it in earnest, but those who use it in its traditional sense risk confusing people, and those who use it in its newer sense risk being corrected by careful readers. In many cases, it’s difficult to tell what a writer means by the word. The good news is that, in either sense, the word has plenty of perfectly good synonyms.