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 loveera 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 clubdance 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.
Kyle is just eating this shit up, huh? What a fuckin' onion.