Joint pain caused by participating in activities with your bros.
Examples of such activities include but are not limited to: Sitting around playing video games for too many hours on end, injuring one's self during a "hold my beer" moment, or passing out in an awkward and/or uncomfortable place or position (like the back seat of a Volkswagen).
"Bruh...my Brositis is really acting up today after jumping out of Chad's window and sleeping in the bushes last night."
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi