Well, today in a blog comment—there was the word again. But today the context was such I really did wish to understand what the commenter meant by that. So I broke down and looked it up.
Google sometimes produces a dictionary definition as the first response to a search, and it produced the following:
Well, here is a pretty kettle of fish. The two definitions of this word are contradictory—and neither has much or indeed anything to do with “no more.” Additional digging into this contradiction produced the following quote from the Oxford Dictionary people (link). They rank high in Word Land even if they are British and hence, well, a little down-the-nose when talking about our uses of the language:
In standard use nonplussed means ‘surprised and confused’, as in she was nonplussed at his eagerness to help out. 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 on the assumption that non- was the normal negative prefix and must therefore have a negative meaning. It is not considered part of standard English.Not part of “standard English”! Looks like we’ve done it again. The comment I read came from a relatively young American. So I still don’t really know what the comment meant. But, it seems to me, my old policy was a good one and needs only a small adjustment. When Agatha Christie uses the word, I can read the meaning as baffled. But when it comes from a young American, I must discretely look the other way.