JACK. A nickname which has, like many others, become a surname. With respect to this appellation, it is curious that meaning, as it originally did, James, from the French Jacques, and Latin Jacobus, it should have come to be considered as a synonym of John. It was usually applied in a contemptuous way, as in Jackanapes, Jackpudding, Jack-a-dandy, Jack-at-a-pinch, Jack-in-oliice, &c.

Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.