Quote:
USAGE In standard use, nonplussed means ‘surprised and confused’:: the hostility of the new neighbor’s refusal left Mrs. Walker nonplussed. In North American English, a new use has developed in recent years, meaning ‘unperturbed’—more or less the opposite of its traditional meaning: | hoping to disguise his confusion, he tried to appear nonplussed . This new use probably arose on the assumption that non- was the normal negative prefix and must therefore have a negative meaning. Although the use is common, it is not yet considered standard. The preferred spelling is nonplussed.


Not yet? My inner prescriptivist simply froths silently at the mouth.