I understand that language is dynamic, and colloquial misuse turns into accepted use, and so nonplussed means unimpressed and disinterested means uninterested, but with this expression I will stand with the pedants and the original meaning.
Also, you can add "droll" to the long list of misused words.
I don't see the point of insisting the original/archaic meaning of a word is 'correct' if, in the English-speaking world we live in today, essentially nobody uses it or understands it in that sense.
[1] I certainly do not think the same about nonplussed, however. I've never heard that word being used for unimpressed before, but it appears to be simply an Americanism; one I hope never catches on elsewhere.
http://www.oxforddictionaries.com/definition/english/nonplus...