To nonplus someone is to perplex them, or in other words, to cause them to be at a loss as to what to say, think, or do.

// The stranger's odd question about the town where my grandfather was born nonplussed me.

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"Motherhood is only somewhat less likely to nonplus the reader than How Should a Person Be? On one level, it's a feminist disputation over art versus maternity—whether a female writer must be a mother or whether she can get away with being just (just!) a writer. But this is also a book about life with a capital L." — Judith Shulevitz, The Atlantic, 10 Feb. 2022

Does nonplus perplex you? You aren't alone. Some people believe the non in nonplus means "not," and assume that to be nonplussed is to be calm and poised, but in fact the opposite is true. If you are among the baffled, the word's history may clarify things. In Latin, non plus means "no more." When nonplus debuted in English in the 16th century, it was used as a noun synonymous with quandary. Someone brought to a nonplus had reached an impasse in an argument and could say no more. In short time, people began applying nonplus as a verb, and today it is often used in participial form with the meaning "perplexed" (as in "Joellen's strange remark left us utterly nonplussed").