The occurrence in which a woman's spandex (or other tight-fitting article of clothing) constrict her labia so independently to such a degree that they appear to be a tiny set of testicles.
"Oh, my, God, Becky, did you see that girl at the gym with the blue spandex??"
when a penis is so small that it looks like there is a crevice between your testicals that resembles a vagina, so hence balls and a vagina makes a ballgina