You don't know Jack (YDKJ) derives from the slang phrase "you don't know jack shit". As an example: Jane Schitt married Jack Schitt and they have 2 children.

One day Jack gets arrested for soliciting prostitutes. Jane told her girlfriend that she thought she knew Jack. The girlfriend said, no pun intended but "you don't know jack".

Women and men see two things when meeting the opposite sex for the first time, 1. Their Looks and 2. A portrait of their character. Now, I use the word (portrait) because a portrait is limiting it cannot tell you what is on the persons mind. Which leads me to a third point, 3. Motive. What is a person's motive for wanting to be with you or doing other activities and until you know what that is you don't know Jack.

Motive is doing something, especially that is hidden or not obvious. The operative word is hidden.

What does a man or woman want with you, sex, marriage, children and what else. Motive is a hidden character not easily reveal by looks or character.

Character is superficial, appearing to be true or real only until examined more closely. Character supports motive but won't reveal a motive alone. Motive is revealed by questioning and observing. Could Jane recognize Mr. Schitt's activity, possibly but his presumed good character may have taken center stage therefore she ignored his time away from home that should have been questionable over time.

She thought she knew Jack because he loved her and was good to the kids but it turns out she didn't.