In the dictionary, a Plagiarist is "Someone who uses another person's words or ideas as if they were his (or her) own." In my dictionary, a Plagiarist is no more than a common thief who steals from you.
If it's yours, put your name to it. If not, attribute it to its rightful owner. Even if it's just a line, a quote, a passage, whatever. If you don't know who the original writer is, then say "Author Unknown".
It's the first, the least and most decent thing to do in any kind of writing.
P.S.: I own this post.