DUCK-RABBIT [in highly exasperated tone]: ___, you are driving me completely BANANAS.
YOUNGER [breaking out in helpless giggles]: BANAAARNAS!!! BANAAARNAS???!!! [pronouncing the word in mocking imitation of her mother]
YOUNGER [continuing in lofty tone]: Or, as Americans say…
DUCK-RABBIT [interrupting in anticipation of what is coming]: Yes, as Americans say ….
YOUNGER: “You are driving me completely INSANE.”
DUCK-RABBIT [nonplussed ]: What?
YOUNGER: “You are driving me completely INSANE.” That’s what an American would say.
DUCK-RABBIT: Oh. [a little crestfallen] I thought you were going to say “you are driving me completely BANÆNAS” [pronouncing the word in mocking imitation of an American accent]
DUCK-RABBIT: Right, I realize that now.
 I happened to look up nonplussed in the OED online just now, as you do, and discovered something peculiar. You could say that I was nonplussed by what I discovered. According to the OED, “nonplussed” has two distinct and opposed meanings. The first, the one I’m familiar with, is “1. Brought to a nonplus or standstill; at a nonplus; perplexed, confounded.” But then check out definition 2: “2. orig. and chiefly U.S. Not disconcerted; unperturbed, unfazed.” Then there is a note: “This usage is often regarded as erroneous: see discussion in etymology.”
Oh you crazy Americans! The OED’s “discussion in etymology” suggests that the American usage resulted “probably as a result of association with other words in non-prefix.” But wouldn’t this only make sense if “plussed,” somehow, had the connotation of being agitated in some way?
Disappointingly, plussed doesn’t actually seem to be a word in its own right, although I think it should be (Try saying “I’m so fucking plussed right now” and tell me it doesn’t feel right). But even as I tried out plussed and found it pleasing, I wondered if it feels right as an adjective describing annoyance because it sonically evokes other words we use to describe vexation. Plussed at once assonantly recalls fussed and consonantly evokes pissed. Or maybe it’s simply that the most frequently used examples of non-prefixes are applied to conditions in which the negated quality is bad: as in non-toxic, or non-threatening.
I remain nonplussed (and, for now, nonbananas).