Nonplussed

/nɒnˈplʌst/ adjective medium

To be nonplussed is to be caught so off guard by something unexpected that you are momentarily unable to respond.

The interviewer was nonplussed when the candidate answered every question with another question.

Memory Trick

Break it into 'non' (not) plus 'plussed' (added up) - the situation doesn't add up in your mind, leaving you baffled.