Earlier quoted context omitted.
Your observation about the root of the word might be accurate, but (in my personal view unfortunately) is not relevant. "Plagiarism" is now used to describe previously published (and in some cases unpublished!) work that is being "recycled", regardless of author, without explicitly noting it to be such. I'm not defending the usage, I'm simply explaining how the term is now used. Language mutates, origins are lost, an…
This is why I'm asking whose definition it is. Whose idea was it to start using this term, which had a useful and important definition for generations, to refer to something else now, and then act as though everyone should just know this? I'd assume it would have had to be somebody pretty important, for their fiat to have such weight. I'd like to know who it was.
It's a concept that needs a name, and when a name turns up it often spreads quickly with no obvious source.