The Collaborative International Dictionary of English v.0.48:

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]