Now I can see how something like "Tessa" would derive from "Theresa", and even how "Sally" might come from "Sarah", and I just found out that "Ike" falls from "Isaac", which is actually kind of cool, but there are several out there that completely elude me, logically speaking.
The ones in question, specifically, are "Jack" and "Jake".
We hear references to the late John Fitzgerald Kennedy, and his brother Robert, as "Jack and Bobby", often by people who would like to think they were friend or kin to those two (but never mind that).
Now, back up a second.
In the etymology of names, is not John a name unto itself? Seriously. Look at it, especially Biblically (and I'm not necessarily a Bible fan, but that's a rant for another week and a half that I won't start now). John is a saint. John is, if I am not mistaken, actually an Apostle. And I fail to see how "Jake" falls from "John". "ay" vs. "ah", you know? Etymologically speaking, it just doesn't match! What are these people thinking?
"Jake" sounds like it could come from "Jacob". Ditto "Jack". "Jacob" is not a name you're going to hear often unless you're traveling in Hasidic or Judaic circles a lot. It doesn't surface.
...unless you're one of the ones who was named, given the propensity for a stuffed shirt at birth, "James". Of course, how "James" came from "Jacob" in the first place was a twisty little maze all its own (Jacob (Heb.) -> Jacobus (L.) -> Jacomus (Late L.) -> mmmyeah, that kinda makes sense, if you're drunk or stupid and drop lots of letters from your words...).
But the point is that "John" is truly sore out of luck as a name, from my point of view. I could never name a kid "John". What do you do with that name? You can't really call him "Jake" or "Jack", not properly. "Johnny"? Well, I guess it worked for Johnny Cash. You could misspell it "Jhon" and take it from there (I know someone who did that with his last name. He's a very good photographer). The only "Jake" I knew was a "John" was John Kelly, who went by "JK", his initials, and everyone called him "Jake".
Not that I'd be wanting to name a boy "James", after all, in spite of the diminutives. You get Jim, Jimmy, Jamie (Scottish diminutive, and yes, it is a BOY'S name and I could shoot the person who decided it was a good name for a girl from all the crap I started getting for it about the time the Bionic Woman came out on TV, different spelling notwithstanding since it was all about the pronounciation), and fun little ones like Jim-O, Jammer, Jim-Bob (go a bit south for that one), but you could also call him "Jake" or "Jack" and be perfectly, rightfully correct in that.
(The reading of how James (English) -> Diego (Spanish) was also interesting, a twisty little passage all its own. Google it. You will discover that "Jaime" is an adoption to the name "James", not a representation of it. Origin of "Jaime" is Iberian, not Spanish, and its meaning is apparently completely unrelated.)
That's about it, since I can pretty much figure out the origins of such names as Babs, Hank, Rusty, Shorty, Sloppy, Sleazy, Shifty, Lefty, Lenny and Bruce...