Where did the expression, (Not telling you jack) come from?

In the days of highway men, the criminals would approach their victims w/ an initial greeting of "Hi Jack". This is where the word "hijack" comes from as people referred to them as hijackers. Thus the name "Jack" became a universal term of unspecific familiarity much like the name John Doe.

" IT ORIGINATES from the prohibition era in America. Supposedly a member of one gang would approach the driver of a rival gang's bootlegging truck with a smile and a disarming 'Hi, Jack!' before sticking the muzzle of a gat in the face of the poor unfortunate, and relieving him of both truck and its alcoholic cargo.THE word 'hijack' has its origins in pre-revolutionary France. Impoverished peasants attacked and robbed aristocrats travelling in coaches through the countryside. The word they employed for this practice was 'échaquer,' which, sharing a common root with 'éjecter' in the Latin word 'eiacere,' meant primarily the physical removal of the aristocrat from his carriage and of his possessions from his person."

https://www.quora.com/Where-does-the-word-hijack-c...

It is a more persuasive way of telling someone to mind your own business.

https://english.stackexchange.com/questions/20437/...