If you read copywritten material and then pass it off as your own you are plagiarizing. Words in a dictionary don't come under that, but I'd bet that if you released a new dictionary that was mostly copied from the old one, most people would consider that plagiarism as well.
I haven't seen any service copy out large block of text enough to make me think it's reasonable to call their output plagiarized.
Meaning, if the LLM I use will only repeat an idea that many someone's have written about, such that it's seen the idea, or parts of that idea many times. Why is that still plagiarism? Or rather, worthy of direct attribution? Or why was I wrong to use the argument about citing a dictionary here?
(I'm aware that a number of people are working on giving memory so AI can quote from pages like wikipedia. But I don't think it's fair to call that "training data")
As you go narrower with a query only one source of truth is available at that point it does plagiarize.
Have you ever seen chatgpt cite or credit it's sources?