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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.
Ah crap, I couldn't stop thinking about that all-night threesome that I had last night, and my penis grew a baculum.
by Ryuseiken September 20, 2006
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