Perhaps the worst of all boners, this boner occurs when one really has to piss. It normally appears during what was originally an uneventful walk to class, or whilst playing a friendly game of Parcheesi with some bomb-ass broads. Only goes away after relieving oneself...
Control your fluid intake, you idiot. No one wants one of these bad boys to pop up in the middle of nowhere.
"Dude you ready to go now?"
"Can't brah; I've got a raging boner of the urinary variety.
"A urinary boner?!? I understand..."