"Nonplussed" is one of those words that historically doesn't have a particularly complicated meaning, but it's one that people frequently misuse.
In fact, the definition of "nonplussed" has become so muddled over time, people often use it to mean the complete opposite of its actual meaning.
Again, the definition of "nonplussed" is pretty simple, so why all the confusion? You could say there's a prefix to blame.
Historically, "nonplussed" simply means to be confused or bewildered to the point where you don't know how to speak or act.
Some readers are now likely feeling a little confused, maybe even nonplussed. That's probably because a lot of people use this word to mean unfazed or not bothered by something, while others use it to mean unimpressed.
Nonplussed" first comes into English from Latin as a noun -- e.g. "he was in a state of nonplus" or "she was reduced to a perfect nonplus." The verb form comes next -- as in to nonplus someone and render them speechless -- and then we get the more familiar adjective form.
Over the years, many people have re-interpreted "nonplussed" to mean "not plussed." That is, if you're not plussed, then you're not phased or excited.
That reintepretation really seems to be catching on. In 2013, the American Heritage Dictionary surveyed its usage panel on this sentence: "The nine panelists showed little emotion during the broadcast and were generally nonplussed by the outcome."
Even though 57 percent of the usage panel rejected the use of "nonplussed" in this sentence, it's worth noting that 43 percent accepted. That means we're pretty pretty split over what this word means.
English Professor Anne Curzan says she's not surprised.
"As the noun and verb [for nonplussed] have fallen out of use, it's very possible for people to reinterpret the adjective," Curzan says. "Other words for 'confused' don't have that negative prefix [non]. Mark Lieberman wrote a nice post on this for Language Log and points out that confused and bewildered have no negative prefix vs. something like 'unpreterbed' or 'nonchalant.'"
Curzan says we're clearly at a point where "nonplussed" is changing. The odds are, she says, that the "unimpressed/unfazed" meaning will win.