An Action Boner is the kind of a Boner that is popped during a movie, tv show, music video, or video game with scenes of action so fuckdamned epic that you can not help but gain an erection.
Guy one: Fuck damn man, did you see that new explicitly violent, cheesy, and outrageously over the top movie!?
Guy two: Fuck yeah man! I popped an action boner.
Man 1: Did you watch 300?
Man 2: Oh my god that movie was so action packed I popped a boner...but...uhh...not in a gay way.