The Collaborative International Dictionary of English v.0.48:

Knabble \Knab"ble\ (n[a^]b"b'l), v. i. [Freq. of knab.]
   To bite or nibble. [Obs.]
   [1913 Webster]

         Horses will knabble at walls, and rats gnaw iron. --Sir
                                                  T. Browne.
   [1913 Webster]