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'