Bonegina is when a girl is so thin, you look down and the inline of her pelvis forms the structure of the vagina. Also can mean a vagina with rows and rows of razor sharp teeth.
"That anorexic girl would be nice to tap."
"No, she has a bonegina."
"What happened to you man?"
"I got my hand stuck in a bonegina, it ate me a-fucking-live."