Grammar Nazi Time. An excerpt from Oxford's American:
1 (of a person) surprised and confused so much that they are unsure how to react: he would be completely nonplussed and embarrassed at the idea.
2 informal (of a person) not disconcerted; unperturbed.
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. It is not considered part of standard English.
1 (of a person) surprised and confused so much that they are unsure how to react: he would be completely nonplussed and embarrassed at the idea. 2 informal (of a person) not disconcerted; unperturbed.
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. It is not considered part of standard English.