Sidebro
A bro who is not really part of your main group of bros, but is chill enough with some of your bros that he can occasionally hang out with your main group of bros.
Hey guys, lets hang out tonight. You can invite any of the sidebros if you want.
sidebro
if your a woman, and have a boyfriend and are talking to another guy behind your boyfriends back. you are his side chick, he is your side bro.
jack is my sidebro but ronald doesn't know!