Once used to describe a woman who was a willing sexual participant for multiple bikies or surfers. Term used during the late 60's through 70's free love era mainly in USA subcultures. More likely to be called a ganger or rooter in Australia
She was the onion in the gangbang behind the surf club dance that night.
That's the onion from yesterday! She's the local ganger and loves rooting.
A satirical insult used to describe people who believe their thoughts to be complex, 'layered', and autonomous, but who are in reality quite shallow. 'Onions' are easily mislead by silver-tongued politicians and disingenuous statistics, much like those who would take The Onion's news parodies at face value.