The interest to a loan on the street or through a bookie. It is common to not pay back the principle but have to pay the "vig" or interest weekly to keep your legs intact. The interest paid in no way lowers the principle. This is why loans are given freely on the streets, as most people borrowing could most likely pay back the entire loan, but can usually come up with the vig every week. This also lets loan sharks get their "hooks into you" if you borrow this money, as you will probably be paying it far longer than expected and after you finally pay off the principle with the vig, you will have often paid more then double or triple of the total borrowed.
borrower - "the vig is killing me and at this rate i am never going to be able to save enough to get this paid off"
loan shark - "exactly. just keep these weekly payments coming or i will break your legs"