A mustache used to tickle ones scrotum. Usually one of the true "bros" scrotum. Or used in action, to tickles ones scrotum, thus becoming a true bro. Sometimes one mistakes this as extremely intrusive, but this is usually the start of a true bromance.
"Is the man with the brostache invited to this party?"
A term of endearment used to describe one's exceptionally close friends. In the event that one may make an absolutely obsurdly idiotic comment, it may be used in a mocking way; as to say that said person would never be a close friend of the accuser. This term was invented by Rhett Smith.
Brostache, did you just whitness that effing hyper-shred?
Yeah, good one brostache, The Locust are talented.
A bro-stache is similar to a "trash-stache." It is a thin layer of peach fuzz type hair that is not fully grown into a full mustache yet. It derives from young latino boys ranging from the ages of 12-20. This age range of young latinos predominantly have these staches and call each other "bros," which is where the term "bro-stache" came from.
Morgan: Woh! Hey Steph, did you just see that kid with the sweet bro-stache?
Steph: Yeah, he looks like such white trash!