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.
      1913 webster

   2. to angle with a bob. see bob, n., 2  3.
      1913 webster

            he ne'er had learned the art to bob
            for anything but eels.                --saxe.
      1913 webster

   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.
      1913 webster
see also:
bob to bob at an apple cherry