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.

nonplussing or nonplusingnonplussed or nonplused to render utterly perplexed by something unexpected; puzzle completely: He nonplussed his questioners by answering in riddles.