nonplus

Merriam-Webster's Word of the Day for March 25, 2011 is:

nonplus • \nahn-PLUS\  • verb
: to cause to be at a loss as to what to say, think, or do : perplex

Examples:
The student's unexpected about-face during the class discussion nonplussed the teacher.

I spent a few days making phone calls, talking to perplexed and befuddled healthcare providers who were absolutely nonplussed by the fact that I wanted to pay them rapidly depreciating American dollars to provide me with healthcare services. -- From Kevin Williamson's 2011 book The Politically Incorrect Guide to Socialism

Did you know?
Does nonplus perplex you? You aren't alone. Some people believe the non in nonplus means not and assume that to be nonplussed is to be calm and poised when just the opposite is true. If you are among the baffled, the word's history may clarify things. In Latin, non plus means no more. When nonplus debuted in English in the 16th century, it was used as a noun synonymous with quandary. Someone brought to a nonplus had reached an impasse in an argument and could say no more. Within 10 years of the first known use of the noun, people began using nonplus as a verb, and today it is often used in participial form with the meaning perplexed (as in Joellen's nasty remark left us utterly nonplussed).