When you are about to engage in sex with someone who is uglier than a barrel full of smashed assholes it is the act of taking their shirt by the bottom and tying it up over their head and then fucking them. Thus, resembling the shape of an onion.
"Lisa has such a hot body, but her face is uglier than shit..." "I think I'm goona have to give her the "Onion Treatment"."
by ed mack September 30, 2006