A bone in occurs during intercourse between a male and a female, when the female has exercised her vaginal muscles to the extent that she is able to force her labia through the base of the male's crotch, absorbing a large portion of his pelvic region.
It can also occur during sex between two males, where the receiving male has exercised his anus enough to perform the act.
Scholars are still unsure whether this came before or after the bone out
I love my girlfriend, but I'm afraid she may want to bone in.
If you're on the road to prison, it may be valuable for your well-being to master the technique of boning in.