To me it means to be confused/surprised and not sure how to react to something
But I've seen it used on here and on other online forums to mean the opposite - so unfazed, not confused, carry on as normal etc
I don't have any problem with language changing, adapting, new uses if words etc, so it may be that I'm just behind the times with this one. But it does leave me confused (nonplussed?
) when I'm reading one meaning and it's only after a few readings I realise that the other person means the opposite