This term is a common euphemism for penis that might make one think there’s an actual bone in a human penis. Of course there isn’t, but our not-so-distant ancestors may have had an actual bone in theirs. Many species of mammals, including apes, do have a bone, called a baculum, that protrudes into their penis for more instant erections.
A baculum from a walrus
I used to be super active on a sex site known as The Smut Shack. Sadly it no longer exists. I was one of the mods on the site and I used the handle of “Boner” for the year or two the site was active. I was totally addicted to the site and in a way I’m glad it’s gone because I spent way too much time on it. If you were ever active on The Smut Shack I would like to hear about it.