/nonplusst/

• 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’.

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Uninterestingly, both uses can be found in this thread: http://drownedinsound.com/community/boards/music/4201439