meaning ‘to perplex’, has inflected forms nonplussed, nonplussing, and in AmE also nonplused, nonplusing. In a recent development, nonplussed has come to mean ‘unperturbed’

‘Out of power?’ asked McCoy, trying to appear nonplussed

D. Kramer-Rolls, AmE 1990

, the very opposite of the accepted meaning, probably by association with the prefix non-, which implies a negative meaning.