Recently, I came to consider the potential ambiguity in the word "nonplussed".  The word originally meant to be confused or perplexed, as in:

"Defense counsel, undoubtedly nonplussed, attempted to challenge the unexpected identification by asking, 'Now, you told me a few minutes ago when I talked to you that this looked like the man but it wasn't the man?'"

People v. Jackson, 187 Cal. App. 3d 499, 508, 231 Cal. Rptr. 889, 895 (1986).  This meaning is consistent with the word's etymology.  It is a combination of the Latin words, non plus, meaning nothing more.  Someone is perplexed when they can think of nothing more to do or say.

Some courts, however, use the word to mean unperturbed, as in:

"The gun jammed. Nonplussed, he herded his companions back into the car and the three men continued their meanderings."

United States v. Bradshaw, 281 F.3d 278, 281 (2002).  Undoubtedly, courts deploying the word in this manner assume that "plus" must mean perturbed and therefore a nonplussed person is not perturbed.