Green’s Dictionary of Slang

1. (Aus.) a second helping of food; thus as v., to take a second helping; also attrib.

1903		Worker (Brisbane) 11 July 7/2: The food was handed to them as if they were convicts [...] it was an offence to back-up their plates for a second helping.
1929		‘F. Blair’ Digger Sea-Mates 19: ‘Our first meal aboard’ [...] ‘Any “buckshee”, Boy?’ he enquired. ‘Yes. Back up yer cart.’ Tom’s plate received another slice of beef [AND].
1946		R. Rivett Behind Bamboo 395/1: Backup, second helping. Backup king, man always chasing second helpings.
1951		Mail (Adelaide) 27 Oct. 10/3: It was big enough for every man to have a full helping [...] and there was more left over for a ‘back-up’.
1953		T.A.G. Hungerford Riverslake 121: There’s three dixies of goulash left over. Why not let them back up until it’s all gone?
1959		Bulletin (Sydney) 30 Dec. 16/1: That stew was a beaut, with plenty for ‘back-ups’.
1961		T. Ronan Only a Short Walk 177: That hungry-gutted mob will swallow theirs straight and be asking for backups.
1971		F.J. Hardy Outcasts of Foolgarah (1975) 25: We fed yer. Right? And we know you appreciate it because you always have a back up of everything.
1972		D. Jenkins Semi-Tough 294: Jim Tom motioned for some back-up Scotches.

2. (Aus./US) multiple rape of a woman.

1965		W. Dick Bunch of Ratbags 169: A ‘square’ was a society person who [...] didn’t like widgies, beer, fights, back-ups or bodgie clothes.
1972		R.A. Wilson Playboy’s Book of Forbidden Words 23: ‘The club just had a backup with the new ginch’; that is, a young lady has just experienced motorcycle-club initiation. The term is of Australian origin.
1989		J. Morton Lowspeak.