Green’s Dictionary of Slang

(orig. W.I. then UK black) any form of oppression or repression experienced by blacks (esp. at the hands of the police); thus under heavy manners, under strict discipline.

1976		Time vol. 108 553/2: ‘We are not going to be ruled by violence but by heavy manners. No one can hold us back.’ [...] Now Manley may find that more than ‘heavy manners’ — slang for discipline — will be required to save the country from bankruptcy and bloodshed.
1983		(con. mid–1970s) T. White Catch a Fire 255: The violent political tug–of–war between the JLP and the PNP that was being called ‘Heavy Manners’.
1992		(con. 1976) S. Davis Reggae Bloodlines 208: Derrick Morgan was called on for a campaign theme song and came up with ‘Under Heavy Manners,’ which was also the PNP election slogan. The phrase ‘under heavy manners’ in Jamaica describes a child being disciplined by its parents.
2001		J. King White Trash 201: He talked about the way hardcore Christians imposed their heavy manners on a pagan country.
2011		J.J. Connolly Viva La Madness 191: The lump he’d left guarding the back door [was] under heavy manners.