A proposed scenario where, during sex, the vaginal muscles suddenly close around the penis, lodging it in the vagina and keeping it stuck there. It's been said that, should this occur, even after the errection dies down, it would require surgery to fix this. No real occurence has happened thus far.
"My boyfriend has this paranoid fear of penis captivus so we rarely have orgasms."
by Bowman June 16, 2002