How did the guy’s name Jack become synonymous with “nothing” as in, You don’t know jack.
Every time I hear this or say it, I want to ask it to the Teeming hundreds and finally remembered.
We’ve got a new guy here at work named Jack. And he doesn’t know jack.
But while he is on the older side, I doubt he’s old enough that they named it after him. 
I always figured it was short for Jack Shit.
Which gives us the question who is Jack Shit and do we know him well enough to be on a first-name basis with him?
Back off, man. I’m a scientist.
Websters online doesn’t have a relevant definition (Is the OED online?), but going thru the associated words (jack of all trades, jack plane, etc.), I get the impression that the word once referred to a journeyman or a journeyman’s level of knowledge. So maybe, if you don’t know jack, you’re barely good enough to be a hand.
I, on the other hand, do know Jack. My father was called that from the day he left the hospital. 
Am I the only one who thought this thread was going to be about the game?
Jack
15. the least bit; anything; JACK SHIT; (in ono negative constr.) nothing- usu. used in negative constructions.
a1973 PFC, US army, US Taiwan Defense Command (coll. J. Ball): Them new guys don’t know jack!
1982 in “J. B. Briggs” Drive-in 1980 he doesn’t do jack about it.
1984 Buckaroo Bonzai (film): You’re not getting jack from me
1986 Kingsport, Tenn; man, aged 32: My father’s said “He doesn’t know jack” for as long as I can remember"
1986 Miami Vice: You don’t know jack about this
1987 National Lampoon (June) 44 He didn’t say jack
1989 S. Lee “Do the Right THing” I aint boycotting jack!"
1993 P. Munro UCLA SLang 2: I got jack for my birthday. . . We didn’t do jack today.
Historical Meanings:
a seaman
term of address to a man whose name is unknown
a boy or man
a Jacksonian democrat
a flapjack
applejack or raisonjack
a jackass
crib notes
locomotive
jackrabbit
lumberjack
money
jackpot
zinc
a blackjack
jack-rollar
syrup
fact (constr. with “the”)
a fortune (to make one’s jack)
holdup or mugging
jack off (vul)
to trifle or fool around with
to maneuver cleverly or unfairly
to use a tanslation in order to pass
to steal/ hijack/ poach
to strike with a blackjack
to serve (time)
to raise or increase
( from the Random House Historical Dictionary of American Slang, v 2)
In other words, I have no idea.
Add to the list Jack Mehoff.
Oh, and in the south, a jackleg preacher is one without a congregation.
John means ‘man’. It’s a generic term. When stuck for a name, they often use John.
Jack is, for some reason, the same thing. It literally refers to a man, or ‘john’.
Wacky, huh?
So it’s just a phrase in the same way that the H in Jesus H Christ means nothing.
“Vyvyan! Where did you get that Howitzer?” “…I found it.”
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