Max says it is "wrong for to be using double negatives."

One assumes this a critique of the construction "jack squat." One assumes that his objection is that "jack" has the same meaning as squat, permitting one to say "He don't know jack" and "He don't know squat" without altering the meaning of either. Not so, learned one. The clause "You don't know jack" is a shortening of "You don't know jack shit" where the last word is omitted for decency's sake. Squat is a colloquial verb meaning "to defecate" and a noun meaning "defecation." Thus, the construction "jack squat" restores the entirety of the clause, using a synonym in the last position. Eh?