To take something from one sorce and use it towards another.
Many folks believe that this metaphor has its origin in 16th-century England, when part of the estate of Saint Peter's Cathedral in Westminster was appropriated to pay for repairs to Saint Paul's in London.
Jacob: I think I'm going to apply for another credit card so I can pay off some of my bills.
David: Robbing Peter to pay Paul, eh?! Just be carful not to get into debt.