Now in common usage among Americans it seems to mean "not perturbed". https://www.merriam-webster.com/words-at-play/nonplussed https://www.merriam-webster.com/

Now in common usage among Americans it seems to mean "not perturbed". https://www.merriam-webster.com/words-at-play/nonplussed https://www.merriam-webster.com/words-at-play/nonplussed