When a girl is extremely prude and no guy is able to get with her, she is said to have an "Iron Vagina." Hence the name, the vag is impermeable to any male body parts (i.e. dick, fingers, etc.)and acts as if there is a extremely thick sheet of iron covering it, warding off any and all brave, unwanted invaders.
-"Dude, did you get with that hot chick last night, she was so gone"
-"Nah bro, she had such an iron vagina it was un-fucking-believable. i just went home and beat off