Green’s Dictionary of Slang

1. (US) a thug.

1929

M. Levin Reporter 169: Rewrite men are rowdies, blackjacks, pencil-swipers, milk-soppers, and subjects for the priesthood to practice upon.

2. (S.Afr.) a black municipal police officer; also attrib.

1966		K.M.C. Motsisi in Post 30 Jan. (Drum) 16: There are the Municipal cops who the township wits call Black Jacks [DSAE].
1973		M. Dikobe Marabi Dance 73: The ‘Black-Jack’ Municipal police did not see them.
1977		J. Sikakane Window on Soweto 25: The search for the ‘illegal’ natives is carried out by the ‘blackjacks,’ the notorious municipal police. This police outfit is nicknamed ‘blackjacks’ because its members are dressed in black from head to toe.
1985		B. Simon ‘Outers’ Born in the RSA 1997 54: Now there’s blackjacks – uncivilized Zulus man, fuckin’ ignorant kaffirs from the farms and they tell us what we must do.
1989		M. Tlali Footprints in the Quag 18: What about the law? What about the whole set-up of locations and superintendents and black-jacks.
1990		R. Malan My Traitor’s Heart (1991) 266: Those black mayors [were] kept in power by ‘blackjacks’ or ‘greenbeans’ – young black men who were given guns, uniforms, and three weeks police training.