Bob

1. To have a short, jerking motion; to play to and fro, or up and down; to play loosely against anything.
2. To angle with a bob. See Bob, n., 2 & 3.
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.