Gristbátian

According to the Old English Dictionary:

gristbátian

to gnash the teeth :-- Gristbátaþ mid his tóþum fremet dentibus suis, Ps. Th. 36, 12, note. [Gristbeatien, Juliana, 69, 17: A. R. gristbatede; and cf. Laym. gristbating.] gristbatian