In standard use nonplussed means ‘surprised and confused’, as in she was nonplussed at his eagerness to help out. In North America, 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. Although the use is common it is not yet considered standard. Similarly, many people would not consider spelling this word with a hyphen, nonplussed, to be good style.
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