That erection heterosexual men get when
A:observing and/or participating in feats of extreme manliness
or
B: while doing or seeing something extremely Bromantic.
A: Hey man, Turn off the wrestling, it's giving me a broner.
B: I was watching the trailer for "Brokeback Mountain", and popped a huge Broner.
When Two dudes are talking about chicks they have fucked and the dude who is listening to the story gets a boner.
Johnson: the other day i space docked this richously hot bitch at some frat party there was mad poop on her vag.
Steve: no fuckin way that just gave me a ragging broner.
The use of boner as a metaphorical term for excitement, with a play on words of bro.
A broner is the excitement a man receives when he's informed of a bro related event that the man is eager to experience. When a man gets a broner, he is not erect it is merely a metaphor for excitement.
Bro #1: "I got us tickets to the Superball!"
Bro #2: "Dude, I justpopped a broner."
A completely non gay experience in which one bro observes another bro having an erection. Following the experience, the observing bro may follow with "congrats on the broner, bro."
dude, this is our first broner. I know you're ready for spring break with that flagpole.