To fall flat on one's face, or to be face downwards and prone.
From the Norse, á grúfu, 'face-down' or 'on the belly'.
My roommate has absolutely no coordination skills and is often found agroof, moaning piteously.
This word is obscure, but I find it quite awesome that there is a single word that represents a rather common phrase. Cue the roommate commenting about how much she dislikes me in 3, 2, 1...