Nonplus is a verb that means to surprise, confuse, or leave someone uncertain how to react ^{1 2 3}. Here are some example sentences:

• The aggressive questioning at the job interview left her nonplussed.
• I was nonplussed by his openly expressed admiration of me.
• Even the experienced doctor was nonplussed when he saw the patient with a chainsaw in his skull.
• While our host and hostess argued during dinner, my wife and I were nonplussed and not sure of whether or not to intervene.