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].
1985		‘Thommo’ Dict. Aus. Swearing & Sex Sayings 91: ONION — A female who is known to participate in group sex or gang bangs. She can take them one after the other, because like peeling an onion, there seems to be no end to her.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Onion. A woman, particularly one who willingly has sex with a group of males.