You have undoubtedly seen these 'Check Cashing' businesses that have a habit of springing up in the declining parts of town. Well, what these leeches do is give the gullible and irresponsible dregs of our society ADVANCES on their next paycheck, at exorbitant interest rates, skirting the usury laws. Once they get their claws into you, you are doomed. Idiots do this sort of thing in order to support their lotto or cigarette habit.
"I'll gladly pay you Tuesday, for a hamburer today", simply put is spending money before you have it. -or- simply mooching.
Basically, someone who uses this or a similar phrase is trying to get something on credit, and may or may not follow through in paying you back.
"Because food debt doesn't have interest."
Fuck you man! Why pay tuesday for...