No, I know what "Grimdark" is. This is something else.
I've come across the term on RPGnet, and apparently it originated on 4chan's /tg/ board (and no, that has nothing to do with the popular image of the board-that's /b/. The rest of it is just snarky self-deprecation, thanks to /b/).
So, what is grimderp, and how does it relate to grimdark?