From Dot Woodsworth's Mind Your Language column in the September 26th, 2009 edition of The Spectator.
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?
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