The Collaborative International Dictionary
Gruf
Gruf \Gruf\ (gr[u^]f), adv. [Cf. Grovel.] Forwards; with one's face to the ground. [Obs.]
They fellen gruf, and cry['e]d piteously.
--Chaucer.
Wiktionary
gruf
adv. (context obsolete English) forwards; with one's face to the ground