"Stuff" is both a placeholder name for mass nouns and the mass noun form of the plural noun "things", a placeholder name itself. Though its implications are in no way obscene, it is considered slightly but actively impolite in formal contexts; due to its sheer ambiguity, it carries the connotation that better specifying the thing or things to which it refers would not be worth the words necessary to do so. Note that it is not the informality of "Stuff" which makes it impolite. Being short, vague, easy to pronounce and easy to construct a sentence around all make for an informal usage pattern, but only the ambiguity makes it impolite. It is a "cheap" word not because it is simple or easy to use, but because it contains less information than contextually equivalent words of a similar complexity. If a formal word is a glass of wine, and an informal word is a can of soda, the word "stuff" is a slushy. It's mostly ice.
"Jack" is a placeholder name for men. While its connotations are far more extensive and complex than the typical placeholder name, their thrust is essentially identical. "Jack X", where X is a profession, is the "prototypical" X, the one you expect to actually run into, as opposed to the "platonic" X which a dictionary describes. "Jack" thus has the connotations of a prototype: cheaper, more familiar, and more practical than the abstraction, but sketchier, colder, and more skeletal than the ideal.
Building on the offensive ambiguity of "stuff", the word "shit" can be used to mean "a thing or things worth only a four letter word in specification". Thus, "Jack shit" means "a thing at once cheaper, less ideal, and less notable than that which is worthy of no name save for the shortest and vaguest obscenity". Consequently, mincings of the oath such as "Jack Diddly", "Jack All" and "Jack Squat" carry the subtle implication that good words are being thrown after bad, and perhaps because of this, in modern usage the mass noun is often simply dropped in the confidence that context alone will ensure the expression is interpreted as intended; not as the proper noun which now solely comprises it, but instead as the short form of an obscene euphemism for an incoherent concept so abstract as to be nearly undifferentiable from the empty nameless fabric of abstraction itself, yet which is unworthy even of being named "the thing unworthy of being named".
EXAMPLE USAGE
Shake: I haven't paid taxes in six years. I am not gettin' busted by a sandwich!
The Voice: Then you must find another with an appetite for insanity.
Shake: Hey. You're dumb. Eat this.
Meatwad: I heard what that boy said; I ain't eating jack.
Shake: You're gonna let this guy scare you? How bad could it possibly be?
The Voice: It's a world of skinless, blood-soaked nightmares clattering from the deep and clattering for the meat of the guilty.
Shake: Oh, come on. The guy's just jealous. Go on. woof it down, dog.
Meatwad: Well I don't know, I mean, is the mayonnaise fat-free?
Shake: You're a dog.
Meatwad: 'Cause, you know, it's not the calories that get you, man. It's the saturated fats.
Shake: [stuffs Meatwad's mouth with the Broodwich] Woof it, you mutt.
Frylock: Meatwad, no!
[Meatwad gets sent to Hell]
Shake: Hey, did you hear what I called him? He can't do jack about it. [Meatwad reappears from Hell] Hey! How'd ya like Mr. Sticks?! He was a real treat, wasn't he?
Meatwad: Yeah... Jerry said you guys had a little run in, but he's a decent guy, I mean, once you get to know him.
Shake: Bull crap! I know that guy was all over you with his axe!
Meatwad: Nah, that don't sound like Jerry. Now, the Jerry I know took me to Merry Christmas... which is a strip club... Merry Triple X-mas... You see what I'm saying? You see what I'm saying?!
Shake: Gimme that sandwich!!
[Shake quickly eats the Broodwich and is sent to Hell]
Frylock: Wow, so you're saying it was fun?
Meatwad: Hell no! That son of a bitch had an axe!
[Cut to hell, where Shake appears next to two stick-men]
Jerry: So, I don't know, she’s like, “move your skulls to the basement, because I got these drapes” I didn’t get that, I’m like, “honey this is work.”
Stick Man 2: Are you serious?
Jerry: I can’t put ‘em in the fucking basement, I mean, and she’s like , you know, “can you put a tarp over them also?” And I just felt like... no... I’m not. God damn it.
Stick Man 2: I got one at home just like it. And I got a kid now, and so—
Jerry: That’s a whole other set of bulls***, I’m sure.
Stick Man 2: So Cathy puts the co-sleeper right next to my preserved brain collection and she wants me to move them 'cause she thinks it’s not hygienic.
Jerry: I don’t understand how there’s a lack of appreciation for that… backlight coming through the glass of the jars that the brains are in. I mean, it just looks cool.
Stick Man 2: You’ve seen that?
Jerry: Why move it? That’s the point of putting it next to the window.
Stick Man 2: Right.
Jerry: And I’m sure you’ve explained that to Cathy, but she obviously doesn’t get that.
Stick Man 2: Yeah. Yeah. There’s no... Yeah, I can’t even argue with her.
Jerry: Ugh. It’s fuckin ridiculous.
Stick Man 2: If I want to move the bed—hey, isn’t that that guy?
Master Shake: And what's with the toilet seat, right?!
Jerry: —oh, what the fuck!?
[Jerry begins to chase Shake, wielding an axe]
Master Shake: Jerry, no! We're cool! We're cool, man!
BONUS ETYMOLOGY FACTS
The word "flimsy" likely has its origin in "flimflam", a contemptuous echoic construction perhaps connected to an unrecorded Scandinavian dialectal word cognate with the Old Norse "flim", meaning "lampoon". Other potential relatives are "film", in the sense "thin flexible sheet of material", and "phlegm", one of the four humors. The Latin word for things relating to phlegm is "pituitary", as in "pituitary gland". The word "Gland" derives from the proto-indo-european word for acorn. The pituitary gland is also called the "master" gland, because it regulates the other glands of the endocrine system, and, by extension, the physiological components of many emotional states and reactions, including affection, passion, and the fight-or-flight response.
The surnames "Parkins" and "Parkinson" are diminutive forms of "Parkin" ("stone"), and mean "born of the stone" or "small rock". Likewise, the surnames "Masters" and "Masterson" mean "born of the master". Thus "Jack Masters" can be read as meaning "No Hormones" and "Flimsy Parkins" as "Pituitary Fossilized".
