There’s so much going on with nonplussed. First of all, if non- means “not” (nontoxic) or “unimportant” (nonissues), what is being negated? What is the opposite of being plussed?
It turns out that nonplus comes from the Latin words non plus, meaning “no more,” and originally referred to a point in reasoning or argument at which “one is unable to proceed or decide,” according to Noah Webster’s definition from 1828. It meant “a state of bafflement or perplexity,” a synonym of quandary.
Nonplussed, therefore, means “perplexed.”
But there is a further point of confusion that can send someone to the dictionary: since the mid-20th century, nonplussed has been increasingly used to mean “unimpressed” or “unsurprised,” and this use, though often considered an error, has made the confident deployment of this word a fraught issue for many. So they need to keep checking in the dictionary.