Green’s Dictionary of Slang

1. (Aus./US prison) one who can be relied on for support, e.g. in a fight.

1979		E. Torres After Hours 6: [H]e got me transferred to Lewisburg. Plenty rice-and-beans pistoleros there. With a li’l backup maybe I can do my stint.
1984		B. Ellem Doing Time 139: And if you do have a fight you’ve got to strike swiftly [...] because if he has brought some mates with him as back-ups very likely you will also have to fight them.
1989		L. Aparvary Legionnaire’s Journey 153: More than a small dose of courage was needed to saunter into the enemy camp without any back up .
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Back-up. Another prisoner who can be called up for support, especially in a fight.
2005		B. Coleman Rakim Told Me 147: ‘I was a one-man team coming into school, with no back-up’.

2. (Aus./N.Z.) retaliation, revenge.

1982		G. Newbold Big Huey 79: It was rare for a dispute to take place without at least talk of a back-up (retaliation) afterwards.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 11/1: back-up n. an act of revenge.