Jack is a nickname for John
I'm sure his real name is John-in-the-Box.
Wake up people! Not that you really care, but Jack is a nickname for John. It drives me crazy that so many people don’t know that. If I had a buck for every time I’ve had to explain that Jack is a nickname for John, I’d be… well, not rich, but I’d have more money than I do right now.
Why do people not know this? It’s not like I’m the only one to go by Jack but have the given name John. In fact some pretty famous people share this nickname.
Jack Kennedy – JFK or John Fitzgerald Kennedy was our 35th President.
Jack Kerouac – Jean Louis Kerouac was a famous writer
Jack Lemmon – John Uhler Lemmon III was a famous actor
Jack Lambert - John Harold Lambert is a former NFL linebacker.
Jack Morris - John Scott Morris was a MLB pitcher.
Jack White – John Anthony Gillis is the lead vocalist for The White Stripes
Jack Welch – John Francis Welch, Jr. was the Chairman and CEO for General Electric
Jack Lord – John Joseph Patrick Ryan was the star of Hawaii Five-O 
Jack Wagner – Peter John Wagner, II is a former singer
Sure the name Jack doesn’t particularly make sense as a nickname for John since it doesn’t shorten the name or make it easier to say. But to me, the name Jack does seem a little less formal than John and more nicknamey. Of course there are a few different stories on how Jack became a nickname for John, but you can check Wikipedia for that answer if you really care.
So now you know, if you didn’t already. Jack is a nickname for John. I didn’t make it up. While I was named John at birth, I was called Jack from that first moment. Maybe soon I’ll join the list above. Here I come Jack Wagner!
**Post Update: Jack Welch made an appearance on 30 Rock last night. Thanks 30 Rock & Jack Welch for furthering the world’s understanding of the nickname Jack.
Does this mean that the more formal name for a donkey is “John-ass” ?
It all depends on how highly you regard the donkey. That talking one in the Bible certainly was impressive.l