the money you have to pay a bum when he buys you liquor or cigarettes (if you're a minor that is). Generally you give them a twenty and let them keep the change, so they get in between 3 and 5 dollars for their services.
A: I've got just enough to buy a bottle!
B: I dont have my fake ID though. Do you have enough to cover the bum tax?
As in a bum has to pay 15% percent of his or her income every month in a money order
this bum forgot to pay his bum tax now he'll get deported to the moon