n. UN'yun fuk - A woman with a hot body, but an ugly face. Derived from the fact that before you fuck her, you'd like to raise her skirt above her head and tie a rope around it to keep it in place, thereby creating an onion-dome shape.
v. - To actually fuck such a woman in such a manner.
Dopey guy, walking behind woman with hot body: "Whoa, I'd like to get sassy with that chassis!"
Same dopey guy, after seeing same woman turn around to reveal her butt-ugly face: "Damn, an onion fuck! I wonder if she'll let me onion fuck her."