G'day Anon,
A mullygrub or mullygrubber is a cricket term.
It refers to a ball bowled that is so tragic that it just rolls along the ground.
The most famous mullygrub ever bowled was by Trevor Chappell against N.Z. in 1981. Google 'underarm bowling'.
A mullygrub is a ball that is rolled along the ground so that the batsman can not score more than one run. It is a sign of a defeated bowler bereft of ideas and resorting to any method to win with no consideration given to the long term ramifications of the actions.
Mully is a variant of muley which refers to cattle with no horns. This shows the lack of danger in the mulligrubber. It is just a 'blah' ball.
Stannum