A harbanje is the first dump of the morning after a heavy night's drinking. Don't pretend that you don't know how satisfying that first crap of the day is. Your body, after hours of heavy-duty punishment, bad dancing and disturbed sleep, is making its first attempt to kick out all the shit (quite literally) that you've filled it with. Once it's plopped out, you feel at least 3000% better, at last ready to face the world outside with a respectable (yet not excessive) amount of fresh-faced enthusiasm.
Please note that, while harbanje can be used in many of the contexts associated with its everyday cousin, the shit, it is not quite as versatile: it is merely a noun and cannot be used as a verb or an adjective. There is much possibility for incorrect usage.
"I'm off to take a harbanje"
"That was one satisfying harbanje"
"I've never been so happy after a harbanje in all my drunken life"