The noun “nonplus” is borrowed directly from Latin, in which “non plus” means “not more.” The original 16th-century usage meant “a state in which no more can be said or done,” and it evolved to mean “a state of being surprised and confused.” In the adjective sense, “nonplussed” officially means “unsure what to say, think, do” or “perplexed.” However, the “non” part of the word has left many American speakers nonplussed. It’s not a standard “non-” prefix that would have a negative meaning, but people want it to work that way. As such, so many folks have used “nonplussed” to mean “not surprised” or “not impressed” that it has become accepted in American English. So, you can use the adjective “nonplussed” to mean “confused” or “not confused” — just look out for context clues.