If a particulary large-breasted woman wears a button-up shirt too small for her bosom and still insists on buttoning all the buttons, a shirtgina occurs; it is the gap between two straining buttons over the front-most swell of the breasts. The gap is akin to the shape of a woman's vagina and if no undershirt is worn, the gap gives a peek into the world of cleavage beneath.
Person 1: Dude! Did you see that girl's boobs?
Person 2: Yeah. That button-up was too small. She totally had a shirtgina goin' on!