And this reaction, I think, hint at why this shift has happened: It will often be unclear if someone is unfamiliar with the word, whether it means perplexed or unruffled, because often the same situation would justify either.
You might be perplexed at the reason someone cares about a situation because you yourself is totally unruffled - being both nonplussed and nonplussed about it... At least a couple of the examples they give are ones where either meaning is plausible.
And so if someone is unfamiliar with the word, it'd be easy for them to infer the wrong thing and as a result associate the wrong meaning for the word going forward.
"I looked at Jim and he seemed nonplussed by the situation." Is Jim acting cool and relaxed according to his character, or is he uncharacteristically flustered? The author knows, the reader might not.
You might be perplexed at the reason someone cares about a situation because you yourself is totally unruffled - being both nonplussed and nonplussed about it... At least a couple of the examples they give are ones where either meaning is plausible.
And so if someone is unfamiliar with the word, it'd be easy for them to infer the wrong thing and as a result associate the wrong meaning for the word going forward.