to perplex or disconcert (somebody) or put them at a loss
The guide was bewildered, nonplussed — Mark Twain
chiefly NAmer used in the opposite sense through misapprehension of non- as non-: to fail to perturb or upset (somebody)
Branson was nonplussed by Elliot's refusal – in fact it constituted the perfect challenge — M Brown
[orig in the sense ‘to bring to a nonplus’; from nonplus2]