When you get a really good feeling by thinking of, or hanging out with, one of your bros.
Like "Lady Boner" it does not have to have anything an actual erection.
It is usually completely non-sexual.
In conversation: "Oh, do you know him to? Just thinking of him gives me a broner!"
In story: Lisa was rocking a raging broner while hanging out with her bro Rob.
Meaning: Lisa was having such a good time while hanging out with her good friend Rob.