1 : unsure about what to say, think, or do : perplexed She was nonplussed, but quickly collected herself, explained that the plumbing had been acting up for days, and told them to pay it no mind. — Diane Ackerman Cedric looked nonplussed. He looked from Bagman to Harry and back again as though sure he must have misheard what Bagman had said.

By the early 17th century nonplus was being used as a verb, with the meaning of “to cause to be at a loss as to what to say, think, or do.” Then, as now, the word is often encountered in its participial form ( nonplussed ), with a meaning that is nearly synonymous with “ perplexed .”