I know what this means: "To pay one debt by incurring another" or other variants of it, but where did the saying come from. I'm not aware of any biblical instance of this.
Deep down I want this to somehow involve Peter, Paul and Mary... :)
The origin comes from the Peter tax and the Paul tax:
The expression refers to times before the Reformation when Church taxes had to be paid to St. Paul's church in London and to St. Peter's church in Rome; originally it referred to neglecting the Peter tax in order to have money to pay the Paul tax.
The Peter tax referred to the tax that people had to pay to fund the building of St. Peter's Church, while the Paul tax referred to the tax that the people had to pay to fund the building of St. Paul's Cathedral.
When the idiom says "rob", it takes that if you don't pay a tax, you are robbing a person (pope, in this case) of what that person rightfully deserves.
Thus, the saying means, not paying the Peter tax in order to pay the Paul tax.
We don't normally like to give links to other resources as StackExchange answers but I think you'll struggle to find a more comprehensive answer than this one:
The lands of Westminster so dilapidated by Bishop Thirlby, that there was almost nothing left to support the dignity; for which good service he had been preferred to the see of Norwich, in the year foregoing. Most of the lands invaded by the great men of the court, the rest laid out for reparation to the church of St Paul - pared almost to the very quick in those days of rapine. From hence first came that significant by-word (as is said by some) of robbing Peter to pay Paul.
In essence the expression is very old, possibly dating to around 1380, but there is ongoing discussion around why the names Peter and Paul were chosen.
The first known appearance of the elemental sense of the proverb "robbing Peter to pay Paul", is from the Science of Cirurgie of Lanfranc of Milan, composed in 1296. In the 1894 Early English Text Society publication of Lanfranc's work, edited by Robert V. Fleischhacker, that appearance is represented as shown here:
...for some medicine is for peter that is not good for paul....
Although OED gives the translation of Lanfranc's use from around 1380, as shown in the Fleischhacker edition, as the first attestation of use of the proverbial sense in English, it is by no means clear from the evidence that Lanfranc's phrase was proverbial at the time he used it; Lanfranc's use may well have simply been an ad hoc collocation employing two common names.
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