Standard definition: Rattled; at a loss for words (see Merriam-Webster)
Common usage: Unworried; unperturbed; cool and calm
Nonplussed is increasingly used to mean almost the opposite of its original definition, which makes it tricky to use. How can you be sure that your audience will understand your intended meaning? If you want to be absolutely unambiguous, it's probably best to avoid.
From Yahoo! News (AFP):
The Pentagon appeared nonplussed by the Chinese reaction, with a spokesman Wednesday saying: "We're not trying to send any message other than a factual presentation of what we know about the state of China's military power."
The reporter means to convey that the Pentagon seemed calm and in control, not that the Pentagon was flustered and taken aback.From Australian Women's Health:
There’s a slight circus atmosphere surrounding Tyra Banks. Not that Banks got the memo – she’s nothing but calm, collected and charming as she settles into a comfy couch to talk to WH, nonplussed by the cameras, security guards and group of fans outside.
The writer is saying that Tyra Banks is unbothered by the cameras and attention, not that she is uncomfortable with all the fuss.**********
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