nonplus (nɒnˈplʌs) vb, -plusses, -plussing or -plussed, -pluses, -plusing or -plused (tr) to put at a loss; confound: he was nonplussed by the sudden announcement. n, pl -pluses a state of utter perplexity prohibiting action or speech [C16: from Latin nōn plūs no further (that is, nothing further can be said or done)]

nonplus / (nɒnˈplʌs) / verb-plusses, -plussing or -plussed or US -pluses, -plusing or -plused (tr) to put at a loss; confound: he was nonplussed by the sudden announcement nounplural -pluses a state of utter perplexity prohibiting action or speech

nonplus (third-person singular simple present nonplusses or nonpluses, present participle nonplussing or nonplusing, simple past and past participle nonplussed or nonplused) (transitive) To bewilder or perplex (someone); to confound, to flummox.

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.