It's no great observation to note that languages constantly change (some, like English, quickly; others, like French, which has a linguistic academy whose role is to regulate how much change is acceptable, more slowly [although it's ultimately ineffectual]). See this short post. But, over the past few months, I've noticed an example of how this is happening right before our eyes -- with the word "nonplussed". It's quickly being turned into its direct opposite.
To "nonplus" means to "baffle or confound". No one, however, uses "nonplus" as a verb, certainly in colloquial American english. But the adjectival form, "nonplussed", is used with some frequency. And, I've noticed that, more and more frequently, it is used to mean the exact opposite of what it (really) means. I never hear anyone use "nonplussed" to mean "baffled, startled, flustered, or confounded". I only hear it used to mean "nonchalant, not bothered, "whatever", etc.
It is, actually, a funny word. First, even though it seems like it should be, it's not a negation of the word "plus". There is no verb "to plus" in english, before which you can stick the prefix "non-". Nonplus, itself (and cognates, conjugates, etc.), is the only form of the word. Secondly, it also, quite confusingly, sounds like its opposite, "nonchalant", which may explain why it is misused so often.
Will be interesting to see how long it will take the dictionaries to change the definition of "nonplussed" 180 degrees. Usage does (eventually) drive meaning.