In standard use nonplussed means 'surprised and confused', as in she was nonplussed at his eagerness to help out. In North American English a new use has developed in recent years, meaning 'unperturbed' - more or less the opposite of its traditional meaning - as in he was clearly trying 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.