Nonplussed means perplexed, confused, or bewildered, but it’s not uncommon for it to be used to mean almost the opposite: composed or unfazed.
President Obama used the word incorrectly in 2008 when asked how his daughters felt about his presidential bid:
I’ve been really happy by how nonplussed they’ve been by the whole thing. (Source: ABC News)
It’s obvious that Obama was referring to how unfazed his daughters were; however, his usage of nonplussed in this way actually means something entirely different.
The word nonplus is Latin for “no more,” as in “no more to say.” When considering the literal Latin translation, it makes sense that it means perplexed. (Someone who is perplexed or confused is often at a loss for words.) But it is easy to see how one might think that plussed means perplexed, and tacking on non makes it the opposite, hence the unfazed usage.