Who is this "Jack" Character?

This topic was brought up today in the locker room before our practice. I thought to myself "who is jack anyway?". What I mean by this is sayings like "this person knows Jack shit". Also, there is a "high jack" you can use by high jacking a plane, is what comes to my mind. We have an exercise of the"jumping jack" and maybe you have a neighbor names Jack. What is just so great about this "jack" piece that we decided to use it vulgarly. While researching a bit on this topic, I think I am about the only one who thinks that this is worth researching because I did not really find anything. :/ . Is it just that Jack is that common in an English name that then has been added to words or phrases ever since? It really makes you wonder. Is it the ease of articulation when you do say something is "jack shit"? I guess we will not really know. I found it extremely interesting!