Short for "Snap Off." An S.O. is a female, usually overweight and unattractive, who finds it difficult to find a male partner who will engage in sexual intercourse with her. When the S.O. finally "gets lucky", she will ride his genatalia so hard, it will snap off under the pressure.
"Oh man, you hooked up with a real ugly chick last night."
"I know had to ice my nuts because she was a real S.O."