A bone in the penis (penis bone, boner) usually found in most mammals, like dogs, cats, and walruses. Humans do not have a baculum(
erections are caused by
blood flow); but girls have the the analogous equivalent- a Clitorus.
In other words, it's the actual missing boner taken from Adam's penis to make Eve(Hebrew doesn't
have a word for penis, so they used rib), and the area where God closed the flesh up is the perineal raphe.
Slang- An erection so hard, long, and everlasting like priapism(but not as long or fatal), that it's mistaken for an actual bone grown in the penis.