Green’s Dictionary of Slang

back up v.³

(Aus./N.Z.) to take revenge, esp. when in a gang.

1978		G. Newbold ‘The Social Organization of Prisons’ (Diss. University of Auckland) 351: The requirement that a person should ‘fight like a man’ is often ignored when a case of standing over occurs, and the underdog is considered justified in evening the odds by backing-up.
1982		G. Newbold Big Huey 53: That afternoon Paul, who was the larger of the two, backed up on Birdie in a decisive manner.
1993		Smith & Noble Neddy (1998) 178: At the time he didn’t have a clue who had attempted to shoot him. He was blaming a group of guys from Melbourne and was all set to go down there and back-up on them.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 10/2: back up 1 to get even, to take revenge upon one who has done one wrong, even over reasonably trivial matters (a face-saving exercise).
2006		B. Matthews Intractable [ebook] ‘Back up, mate. You’ve got to back up on each one of them. Take them out altogether or take them out one at a time’.

SE in slang uses

In compounds

back-up pills (n.) [they supposedly SE back up one’s potency or make seduction easier]

aphrodisiacs.

1984

Partridge DSUE (8th edn) 38/1: since ca. 1910.