Green’s Dictionary of Slang

1. mass sexual intercourse with a single woman.

1969		Sydney Morning Herald 16 July 13/4: When he had passed the circle of men, he knew an ‘onion was going on’ [...] The court was told on Monday that the expression ‘onion’ meant a girl was available for sexual intercourse with two or more men [GAW4].
1976		Southerly ii 136: I would like to win his confidence so that he would admit me to their brotherhood and to rites that I could write about, the inside story of onions, gang-bangs, pack rapes [AND].
1980		B. Hornadge Aus. Slanguage 191: Onion is a modem term, not recorded by Sidney Baker, and refers to the practice of bikie gangs in common sexual sharing of one of the girl-followers or of an outsider unwillingly abducted for the occasion.
1984		National Times (Sydney) 30 Nov. 17/2: The girl alleges the traditional on-i-on or onion as it is sometimes incorrectly pronounced, by a group of motor bike riders in a disused gravel pit after several hours of riotous drinking [GAW4].
1991		R.G. Barrett Between the Devlin 113: ‘Gettin’ into fights and gang bangs. Onions or whatever they call them’.

2. the woman who has sex with two or more men in such a session.

1978		Weekend Australian Mag. (Sydney) 1 July 3/8 Woodhouse told the man the girl was to be the ‘onion’ for the night... He understood that an ‘onion’ in bikie jargon meant a girl having repeated intercourse [AND].
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Onion. A woman, particularly one who willingly has sex with a group of males.