One of many variations of the nickname 'brother'. Brother begat bro, which begat brougham. Brougham has been further modified to create such nicknames as broughamsky.
A brougham is typically one of your close-but-not-closest friends, or a friend of a friend. It can also be used in emergencies when you forget someone's name, e.g. dude, chief, etc.
Almost nobody who uses it knows that it's actually spelled brougham, or that a Brougham is a Cadillac.
"Hey, what's happenin' brougham?"