Etymology

I have often pondered the origin of the word "grim."

Yes, I am a nerd.

Really, what I want to know is if it became a word before or after the Brothers Grimm published those ominous tales about cannibalistic witches eating small children and hunters who surgically remove grandmothers from wolf stomachs. 

Is it just a fun coincidence that their name was so appropriate? Or did people read the stories and then start saying, "Ooooh, that's so Grimm!"

Wikipedia tells me that the Brothers Grimm also did some work on the German dictionary, so maybe they sneaked in a little propaganda there...