Often used as a term of endearment between two bros who are into hardcore stuff: music, extreme sports, excessive drinking, or just brain-dead surfers with horrible judgement. These individuals are usually too full of adrenaline and not enough synapses.
Brain-dead surfer 1: "Whoooaa! What up brohan?!
Brain-dead surfer 2: "Not much brohan!"
Brain-dead surfer 1: "How gnarly is the wake brohan?"
Brain-dead surfer 2: "Well, I figure there's only one way to find out brohan!!"
Brain-dead surfer 1 & 2: "Time to...surf...that...CORAAAL!!"