Where did the nickname of Jack for the given name John originate?

Answer 1

Although it may appear at first glance to be derived from the French Jacques—which is cognate with the names James and Jacob—native speakers of English would almost certainly associate the name "Jack" with "John," rarely with "Jacob," and probably never with "James."

Rather, it is believed that "Jack" is the result of adding the French diminutive suffix "-kin" (much like the German "-chen") to the name "Jean," which is the French form of the English "John", or "Jan," which is a shortening of the German "Johann."

Answer 2

The boy's name Jack is of Old English origin. Name based on John (Hebrew) "the Lord is gracious", or Jacques, the French form of Jacob (Hebrew) "he who supplants".