How is it that Jack is a nickname for John?

Question

How is it that Jack is a nickname for John?

Answer 1

I used to always wonder why they called my grandpa Jack when his name was John.  Well I asked him once and he said that way back when, when the French were running shop in England, John in French is "Jaques", and at that time the country was bilingual, so yu had a bunch of Frenchies running around calling John Jaques and there you have it.  It just kinda stuck(for a few hundred years).

Answer 2

Jack is short for Jackin, an alternate form of Jankin, a English diminutive form of John, the English form of a Hebrew name, Yohanan, Johanan, or Yochanan' ("God is gracious")

Answer 3

Yes --- such as John F. Kennedy being called Jack