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.
   [1913 Webster]

The Jargon File (version 4.4.7, 29 Dec 2003):

gribble
 n.

    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.