Peter

Peter masc. proper name, O.E. Petrus (gen. Pet(e)res, dative Pet(e)re), from L. Petrus, from Gk. Petros, lit. "stone, rock," translation of Syriac kefa "stone" (Latinized as Cephas), nickname Jesus gave to apostle Simon Bar-Jona (Matt. xvi:17), historically known as St. Peter, and consequently a popular name among Christians (e.g. It. Pietro, Sp., Port. Pedro, O.Fr. Pierres, Fr. Pierre, O.Fr. Pierres, etc.). Slang for "penis" is attested from 1902, probably from identity of first syllable.

The common form of this very common name in medieval England was Peres (Anglo-Fr. Piers), hence surnames Pierce, Pearson, etc. Among the dim. forms were Parkin and Perkin. To rob Peter to pay Paul (1510s, also in early 17c. French as descouvrir S. Pierre pour couvrir S. Pol) might be a reference to the many churches dedicated to those two saints, and have sprung from the fairly common practice of building or enriching one church with the ruins or revenues of another. But the alliterative pairing of the two names is attested from c.1400 with no obvious connection to the saints:

Sum medicyne is for peter þat is not good for poul, for þe diuersite of complexioun. [Lanfranc's "Chirurgia Magna," English translation]