When we say we don't know jack about something, it's not immediately clear that we're toeing the line with taboo territory.
Allow us to explain.
In this phrase, "jack" is a shortening of a compound. The second part of that compound is a taboo word. Since That's What They Say is generally a family-friendly show, we'll let you guess what it is on your own.
The "jack" part of this phrase means "nothing" and comes from the use of the name Jack to refer to an everyday man. It's the same "jack" in words like "lumberjack."
What's interesting about the expression "you don't know jack" is that it means the same thing as "you know jack." Both phrases mean you don't know anything.
This phrase is similar to "you don't know squat" which contains another family-friendly word for excrement. We'll admit, we had no idea "squat" fell into that category, but it makes sense when you think about it.
The use of "squat" to mean "nothing" or "nothing at all" is recorded as early as the 1930s. It appears to be a shortening of "doodley squat" or "diddley squat."