• adjective 1 surprised and confused. 2 N. Amer. informal unperturbed.
— USAGE In standard English nonplussed means ‘surprised and confused’. A new meaning, ‘not disconcerted; unperturbed’, has developed recently in North American English, probably on the assumption that the prefix non- must have a negative meaning; this is not yet accepted as standard usage.
— ORIGIN from Latin non plus ‘not more’.
Uninterestingly, both uses can be found in this thread: http://drownedinsound.com/community/boards/music/4201439