"Jack" is the nickname for "John".
Listened to the podcast, "Stuff you should know". One short episode discussed about nicknames. I learned that "Jack" is the nickname for "John".
Link: "Short Stuff: Nicknames" 8 May 2019. Stuff Media LLC. 19 May 2019
The history back to Medieval time. "John" was first called as "Jankin" and then some people also called it as "Jackin". And then "Jackin" became "Jack". Therefore, Jack is a shortening of "John".
Interesting Story ~
