Nonplussed defines its own antonym?

I recently encountered this word while reading an article and found that its two basic definitions are "Bewildered" and "Unfazed." How can the word mean both these things as they seem to be direct opposites of each other?

Should the sentence in which it's used always explicitly reveal the intended meaning?

And while many of us might be a little taken aback if Mom showed up at our offices, Secrist is utterly nonplussed, even happy about it. source

In the above sentence, it's obvious that Secrist is "unfazed" without even knowing the definition of nonplussed.

Instead of adding clarity to the meaning being conveyed, "nonplussed" seems to just add confusion. Then again, maybe I'm missing something. Thoughts?

My first post here so please edit/re-tag as appropriate.

Solution 1:

Merriam-Webster's definition of nonplussed offers faze as a synonym:

transitive verb : to cause to be at a loss as to what to say, think, or do : perplex

Synonyms: abash, confound, confuse, discomfit, disconcert, discountenance, faze, fluster, mortify, embarrass, rattle

Perhaps more importantly, Wiktionary preempts the 'unfazed' definition as proscribed:

proscribed — Some educators or other authorities recommend against the listed usage.

and goes on to say:

In recent North American English nonplussed has come to mean "unimpressed".1 In 1999, this was considered a neologism, ostensibly from "not plussed", although "plussed" by itself is not a recognized English word. The "unimpressed" meaning is not considered standard usage by at least one authoritative source.

Solution 2:

As noted in another answer, wiktionary says the usage of nonplussed to indicate unfazed is "proscribed". Similar advice, plus some background, is given in a randomhouse "Mavens' Word of the Day" webpage. Reader 'Tim' comments on misuse of the term in the New York Times; writer 'Carol' quotes from The New York Times Manual of Style and Usage:

"Nonplused does not mean fazed or unfazed. It means bewildered to the point of speechlessness."

She then comments,

...perhaps The New York Times copyeditors should pay more attention to the basically sound advice in their usage book. The fact that this book takes on the topic shows that the meaning of nonplussed has indeed become an issue; the previous edition did not have an entry for this word. Incidentally, the spelling nonplussed is more common than nonplused, at least in American English. The double s occurs because the second syllable is stressed.

An oxforddictionaries.com entry for nonplussed says

North American informal not disconcerted; unperturbed.

and (with some speculation elided)

In North American English a new use has developed in recent years, meaning ‘unperturbed’—more or less the opposite of its traditional meaning—as in he was clearly trying to appear nonplussed. This new use probably arose [...]. It is not considered part of standard English.