Green’s Dictionary of Slang

1. (Aus./US) to have serial sex with a woman.

1989

J. Morton Lowspeak.

2. (N.Z. prison) to reoffend, to repeat an offence.

1982

G. Newbold Big Huey 244: back up (v) Repeat, reoffend.

3. (Aus.) to re-enlist.

1989		R.G. Barrett Boys from Binjiwunyawunya 59: ‘I met him the second time he was over there’ [i.e. Vietnam] ‘Yeah. Eddie backed up a second time, didn’t he? With the Yanks’.
2006		B. Matthews Intractable [ebook] After the Middle East campaign the old man backed up to fight in New Guinea.

4. (Aus.) to return, e.g. as a visit, to regain a profit, etc .

1989		R.G. Barrett Boys from Binjiwunyawunya 94: I wouldn’t mind backin’ up at the Tai-Ping for another lash at those muddies.
1966		J. Holledge Great Aust. Gamble 23: He backed up again in 1953 when Carioca took the Sydney Cup [ibid.] 59: numerous shrewd-faced little ragamuffins who kept backing up for another collect as long as the pay-out went on.

5. (Aus.) to volunteer, to choose.

2006

B. Matthews Intractable [ebook] I backed up to fight Jimmy the next night.