Words and meaning. This talks about how words come to mean something else. Nonplussed was "not knowing what to do". Now it is becoming "unruffled". But it raises a question. Do words evolve by mistake, or do they evolve because we need to say something and we don't have the right word for it? Unruffled, unaffected, unimpressed - for me all have a sense of ignoring a situation. Nixon was unaffected by student protests…
1. We are looking to express a emotion but want to empathize it more. Existing words get "used up" because we tend to inflate the strength. This is how we get words like "sick!", "awesome" (which changed meaning quite a while ago). Either new words get created or existing ones get taken over.
2. Some people literally don't understand the original meaning and their usage takes over. Supposedly this has happened with "literally".
My guess is that "nonplussed" is experiencing a little of both. Also "unruffled" IMO sounds terrible to modern ears (saying that as a ESL person though). Sounds like someone got out of bed and their hair is still excellent.
Edit: after talking about this with my author wife, I think "unruffled" and "nonplussed" also mean sightly different things to me. "Unruffled" acknowledges that there might be reason to care. "Nonplussed" to me is a lot more dismissive. Which kinda makes it fit my reason 1 for me.