Technically speaking, you did - both in the academic sense of using something that's not yours without citing it and in the more legal sense of potentially violating copyright.

In practice, I don't think anyone is going to care too much. If you have the ability to edit the chapter, or attach a note, you might want to add a disclaimer saying it's from f_w. If you have too much time on your hands, you might want to see if you can track down the original post.

It's an idea that was posted (presumably for public use) on a website. I rather doubt there will be Massive Drama because you used it.