When you sprinkle bread crumbs on a sweaty asshole and then you proceed to fuck them in the ass. The result is the scent of onion rings coming from the asshole.
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!!