The act of taking a relationship with a male friend to the next level. This can be something as trivial as adding a friend to your buddy list on AIM once you realize that you actually give a crap about talking to him. This could also be something as significant as giving your bro an actual promotion on the job front.
How'd you get that KM position?
Well the GM, Mark, is my boy, he gave me a bromotion.
A guy who is bromosexual is totally straight. In fact he will punch you in the face if you say that he's gay. He's so totally straight that he has sex with tons of chicks... sure his bro might be in the room with him, maybe videotaping it (with lots of close-ups of the penis)... or doing the same girl at the same time... with their penises touching...
So what if he's always slapping his broham's ass... and always hangs out in the showers at the gym... and yeah, maybe he was in a few circle jerks in middle school... and sure he puts his penis and/or testicles on his friends' faces every chance he gets when they're passed out drunk... and sure that frat initiation thing was a bit weird, but...
when guys are really good friends, and they make jokes or talk to/about each other in ways that are almosthomosexual, but not quite.
as in:
bromosexuality-
bro1: "shit man that hurt."
bro2: "that's not what you were saying last night"
bro1: "dude... i told you not to tell anyone about that."