The Latin phrase non plus, “not more, no further,” entered English as a noun with the following meaning:
A state in which no more can be said or done; inability to proceed in speech or action; a state of perplexity or puzzlement; a standstill.
As a verb, to nonplus means, “to bring to a nonplus or standstill; to perplex, confound.”
In modern speech, the verb is most commonly seen in the participle form nonplussed. Until about the 1960s, nonplussed was used with only one meaning:
Brought to a nonplus or standstill; at a nonplus; perplexed, confounded.
Since the 1960s, nonplussed has taken on another meaning for some English speakers:
Not disconcerted; unperturbed, unfazed.
The OED mentions this second meaning in its entry for nonplussed, labeling it “chiefly American.”
The recognized authority for American usage, however, does not list this second meaning of nonplussed. It doesn’t even have an entry for nonplussed. The verb nonplus is defined this way:
“to cause to be at a loss as to what to say, think, or do; reduce to a state of total incapacity to act or decide.”
Merriam-Webster gives three examples:
this turn of events nonplusses me
nonplussed by the disclosure
for a moment the girl was nonplussed
The reversal of meaning from perturbed to unperturbed has arisen from the mistaken idea that the non- in nonplussed is a prefix, like the non- in nontoxic. Some speakers seem to think that nonplussed is formed by adding non- to the hypothetical root plussed.
US speakers do use nonplussed with the original meaning:
May 16, 2015 – When invited to come here I was flattered, but a bit nonplussed.—Charles Gibson, US journalist, 2015.
Many consumers nonplussed, confused with latest tech, survey finds—Automotive News, (published in Michigan), 2015.
However, most of the recent examples of nonplussed that I have found in US sources use it in the sense of unfazed, unimpressed, or unmoved. For example:
Uber Driver Nonplussed After Giving Jeb Bush Ride in San Francisco—NBC News. (According to the article, the driver was unimpressed.)
“I’m in first place by a lot, it seems, according to all the polls,” Trump says, in his New York accent, with his usual facial expression: a sort of perpetually nonplussed duckface, like he is continually being impressed with himself anew.—Atlantic Monthly, 2015.
Nonplussed, Colbert has kept up his usual antics.—CBS News, 2015.
Unlike self-antonyms like dust and sanction, whose meanings are usually clear from context, nonplussed is a source of ambiguity.
The phrases “dusting the furniture” and “dusting the crops” require no additional qualification; the different meanings are immediately apparent.
A statement like “the defendant was nonplussed,” however, means one thing to one English speaker and the opposite to another.
Take, for example, this statement in the clinical study of a disturbed teenager:
He appeared nonplussed when the issue of the family cat was raised.—Francis D. Kelly, The Assessment of Object Relations Phenomena in Adolescents, Routledge, 2014.
The boy was in treatment because, among other disturbing behavior, he had killed the family cat. When I read the sentence, I understood it to mean that the mention of the cat caused the boy to exhibit signs of confusion. On second thought, I realized that the author may have intended nonplussed to mean unmoved.
Authors of serious works cannot afford to use nonplussed without providing context clues to indicate which meaning is intended.
The following examples provide such clues:
In an interview with the CBS news magazine “60 Minutes,” Steve Schmidt described Palin as “very calm — nonplussed” after McCain met with her at his Arizona ranch just before putting her on the Republican ticket. (nonplussed=unperturbed)
Vance appeared nonplussed and genuinely surprised that such large political questions had been raised by the memorandum. (nonplussed=agitated)
O’Donnell appeared to be nonplussed by the wording of the first amendment, repeatedly returning to the subject and sounding incredulous after her Democratic opponent Chris Coons attempted to explain it to her. (nonplussed=confused)
Because nonplussed no longer conveys the same meaning to all readers, writers must be thoughtful in its use. Accompanying the word with context clues is one option. Choosing a different word entirely is another.
Some options for the meaning originally conveyed by nonplussed: perplexed, confounded, disconcerted, upset, agitated.
Some options for the reversed meaning of nonplussed: not disconcerted, unperturbed, unfazed.
So this opposite meaning stems from nothing but ignorance, and folks using words they did not understand without taking the effort to kearn. That’s the kind of drift that needs to be corrected whenever it occurs and stamped out of existence. If you need to use a synonym to explain the word even in context, the word is worthless. Redundant. Ruined.
I’m with ApK on this 100 percent. This is a case in which “chiefly American” means “chiefly wrong.”
I don’t know how they figure it’s chiefly American if the American “authority” (okay, it’s juts MW) doesn’t even recognize it but the British one does. I guess that makes chiefly American mean entirely British. I think it is simply a case of ignorant assumption that a given word means what it seems like it means because it sounds like another, sometimes completely unrelated, word. E.g. noisome means noisy or noise-related somehow, fortuitous means the same thing as fortunate, crapulence has to do with that, a nimrod is an idiot, etc. Nonplussed is somewhat outstanding because it actually means the exact opposite of what it is erroneously thought to; like peruse and scan in that way.