when a guy is dancing behind an unknown girl , for example in a crowded disco or pub , and He is trying to put his pelvis as close,of the girl's bottom as, he can, following the rhythm of music while He is faking to dance.
Girl: She was onioning so hard on my ass last night! I thought it was gonna go numb!!
When you shoplift from the supermarket by scanning items through the self-servicetill as an onion (the lowest price item).
In a shameful display of self destruction, depression and sickening greed (potentially malice), pitiful celebrity TV 'chef' Anthony 'The Abhorrent' Worrell Thompson was caught literally red handed onioning a bottle of red wine and some camembert. His family are dutifully shocked.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"