Nope, you can't plagiarize your own content.
The word "quadragonal" is the only word with "dragon" in it where "dragon" is not a root word. That makes it awesome.
Wikipedia definition of plagiarism mentions the copying of another author's work, so are you sure it's not called something else?
Not to go too off topic, but Zekurom isn't wrong--in academics, you have to state if you're reusing a paper. It's considered a violation of ethics if you resubmit a paper, although I'm not sure it's expressly considered plagiarism. But for our purposes (and the purposes of more general "society,") a writer's work is his or her own, and a writer can't steal from themselves.
But someone could copy a fanfic or another authors work and then paste it on another site. How would you track that?