Where you get stupid-ass hobos and pay them little or no money to fight and do other stupid shit for your own sick amusement.
Paying on to tattoo BUMFIGHT across his knuckles or his forhead for a couple slugs of whiskey.
The act of inserting some sort of baton i.e. drumstick, chopstick, dildo etc. etc. up your arse, leaving a good length sticking out. Your opponent does the same and you hit each others baton with your own to make you opponents fall out. The first one to lose his/her baton is the loser, allowing the winner to run his baton under the loser's nose getting a good whiff.
Quote Dirty sanchez 'You put a jellyfish on my T-Shirt, cracked an egg on my face and wrote BumFight(s) on my forehead'
Also 'I challenge you to a bum fight - a drumstick shall be my weapon of choice'