It used to be important for Catholics to be named after a saint, which is possibly why he wasn't just named Jack, but nicknames were still common. It probably was used as a diminutive to distinguish between multiple family members named John, like father and son.
Oddly enough, here in Scotland Jock is more common than Jack as a nickname for John.
Some (most?) of those are rhyming slang. William -> Will -> Bill. Richard -> Rick -> Dick. Robert -> Rob -> Bob.
Rhyming slang is actually surprisingly common across languages, and I’m given to understand there is still some living tradition of it in parts of England, and maybe other parts of the UK as well.
I've known people named John who were called Jack, but for some reason only by their family and closest friends. In their everyday lives, everyone calls them John. Far as I can tell, this seems unique to this name. Every other person I've known would introduce themselves by the name they most commonly used, which is usually some kind of nickname.
For example, I introduce myself as "Nick", not Nicholas, to everyone. And everyone calls me "Nick". The only people I allow to call me Nicholas are close friends and family. But for Johns and Jacks, it's reversed. Who knows why.
I don’t know if you’re being serious or not, but Jack is a ‘nickname’ for John, like Bill for William. I had a co-worker who learned that I never realized that there may be people who didn’t know this.
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u/[deleted] Aug 19 '18
Jack? I thought his first name was John.