When I was at school I remember two kids that were called Jack. Interestingly neither of them were really Jack. One was Mark and the other was Andrew.
I believe that Mark had been called Jack because his brother shot one of his eyes out with an air-rifle and so he was then nicknamed One-eyed Jack, or Jack for short.
The other was a Casanova and always chasing skirt. Jack was either for Jack-the-lad or Jack Russell, I never discovered which.
One-eyed Jack was the scrum-half (and captain I think) of the rugby team that I played on. That story is better told here: The Glorious Sixth
Long live Jacks that are not Jacks!