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You may have been wondering how it is that someone named John is likely to be called Jack as a nickname, as was the case with John F. Kennedy. Fair question. This derives from from a linguistic quirk of the old French (and the old Scottish), that of appending to one-syllable names as an affectionate extender, "-kin."

Now, this really all starts thousands of years ago, when the Hebrews decide to use the phrase "HaShen" (literally "the name") to refer to God outside of prayer, the better to avoid accidentally blasphemously speaking the name of God ("Yahweh") in in inappropriate moment. From this came the name "Yochanan" -- literally "the name (God) is gracious." The Latin spelling of this came out as "Ioannes," but (as Indiana Jones had to recall in Indiana Jones and the Last Crusade), in Latin the J is spelled with an I. And so, inversely, when this Latin name was Anglicized in became Johannes (the "h" gets thrown in to highlight the syllable split). Which gets shortened to Johann, then to John -- and transplanted to France, as Jehan and Jan; so in France it becomes Jankin (and in Scotland, Jonkin), shortened again to Jack, and Jock, the former of which is carried back to merry old England.

So common was John among a Fourteenth Century names, and therefore Jack as a nickname, that soon any male person whose name you didn't know, you could just call them "Jack." If you needed help lifting something heavy, you'd use a jack -- which is why you still do. With so many Jack's running around, if you were lacking in knowledge it would be said that you don't know jack, you must not know anything. The appellation was so frequent among sailors that traditional sailor's uniform became known as the cracker jack, for which a confection would later be named. And, since one wouldn't presume to call a nobleman a common nickname like "jack," the term also came to mean a low-born person, even a knave. So the face card below a Queen and above a ten, traditionally called a Knave, also came to be called a jack. And when cardmakers started printing the letter or number of each card on the corner, K was taken by King, so Knave was out and Jack was in to stay. And so now, my friends, you know Jack.

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Note: A fellow noder asks, 'is this where the phrase "every man jack" came from?' That is not one that I looked into, but it's a safe bet. Naturally, Jack of all trades does have such an origin, and a jack in the box is named for a particular fellow, real name John. Jacking off, on the other hand, seems to be just a bastardization of jerking off.