The Collaborative International Dictionary of English v.0.48:
Gribble \Grib"ble\, n. [Cf. Prov. E. grib to bite.] (Zool.)
A small marine isopod crustacean (Limnoria lignorum or
Limnoria terebrans), which burrows into and rapidly
destroys submerged timber, such as the piles of wharves, both
in Europe and America.
The Jargon File (version 4.4.7, 29 Dec 2003):
Random binary data rendered as unreadable text. Noise characters in a data
stream are displayed as gribble. Dumping a binary file to the screen is an
excellent source of gribble, and (if the bell/speaker is active) headaches.