Jack pipped Mohammed as the most popular boy’s name for babies born last year. There were 8,007 Jacks and 7,576 Mohammeds, or similar spellings. To me Jack is a pet-name for John — a hypocorism, as the grammarians rejoice to call babyish versions of names. You wouldn’t baptise anyone Jack. There is no St Jack. (There is, I think, a St Ernest, from Zwiefalten in Germany.) The French name Jacques is their version of James. So, how did Jack become the English familiar byname for John?
There are two fierce schools of thought. Some assert that Jack is indeed the same word as the French Jacques. The trouble is that, from its earliest appearance in English in the Middle Ages, Jack has been used as an alternative to John. As people were strongly aware of the saint whose name they bore, it would hardly do to use the name of one to signify another.
The other theory was championed by E.W.B. Nicholson (1849-1912, a denizen of North Oxford, a keen cyclist and the librarian of the Bodleian for 30 years. In The Pedigree of Jack (1892), he argued that Jack was nothing but a development of Jankin, a diminutive of John. Someone in the early 15th century was of the same opinion, for a Latin document from Canterbury written in 1414 explains how the English treat Latin names, so that they put ‘pro Thoma Tomme sive Tomlin, pro Iohanne Iankin sive Iacke’. When Jack first appeared, in the spellings Jakke, Jacce, Jacke, it was pronounced as two syllables.
Jack was taken as the name for the common man, as Gill was for the common woman.