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.