Green’s Dictionary of Slang

of police officers, brutal, in full strength.

1960		E. De Roo Big Rumble 90: This was the neighbourhood where the police concentrated their headbusting force with a man on every lamppost for a long while because of some severe beatings and muggings.
1977		B. Jackson Killing Time 232: The one I heard talking to you yesterday, he had a good job before, in the old head-busting days.