etymological question: "jack shit"

[Douglas G. Wilson]

Spears' dictionary gives "jack shit" or "Jack Shit" as "a worthless
good-for-nothing". Possibly this is a secondary development?

There are some parallels: "every man jack" = "every man". [Also consider
the synonymous "every swinging dick".]

"Jack" = "penis", "copulate [with]", "masturbate", among more than 20 other
things.

"He doesn't know jack" = "He doesn't know shit" -- but perhaps this
equivalence is transferred not from "Not worth [jack] shit" but from
"Didn't do [jack] shit", another popular favorite. If this is the case, one
might consider the following equivalences:

Q: "What did you do all day?"

A: "Jack." = "I didn't do jack." = "I did jack." = "Jack shit." = "I didn't
do shit." = "I didn't do anything." = "Jacked off." = "Jacked the dog." =
"F*cked the dog." = "F*ck all." = "Fanny Adams." [another personal name:
perhaps a friend of Jack Shit!] = etc., etc. [I can personally vouch for
the use of each of these; probably you can too.]

Other possibilities would include "jack" = "donkey" but I doubt this one,
partly because both "jack" and "shit" are stressed in common use in my
experience.

Whatever the origin (and I'm curious as to what it might be), "jack shit"
acts as an intensified form of "shit" (or of "jack"?) by virtue of having a
superfluous or nonsense word attached, as with many other expressions:

Don't know shit > Don't know jack shit
Can't drive worth dick > Can't drive worth donkey dick
Don't give a f*ck > Don't give a flying f*ck
Don't give a hoot > Don't give a hoot in Hell
Didn't do squat > Didn't do diddly-squat
Not worth a cent > Not worth a red cent

etc., etc.

-- Doug Wilson