Source : Webster's Revised Unabridged Dictionary (1913)

Bob \Bob\, v. i.
   1. To have a short, jerking motion; to play to and fro, or up
      and down; to play loosely against anything. ``Bobbing and
      courtesying.'' --Thackeray.

   2. To angle with a bob. See {Bob}, n., 2 & 3.

            He ne'er had learned the art to bob For anything but
            eels.                                 --Saxe.

   {To bob at an apple}, {cherry}, etc. to attempt to bite or
      seize with the mouth an apple, cherry, or other round
      fruit, while it is swinging from a string or floating in a
      tug of water.