The noun jack has many varied meanings. It can, for instance, refer to a category of playing card; a device used to lift something heavy; a small opening where something connects with a wire to something else; a small six-pointed object used to play a game; in plural form, that game itself; anything at all—a use that typically appears in negative statements: you don’t know jack.
The word jack has been with us since the days of Middle English, when Jacke was used as a familiar term of address for a social inferior. Even then it was also a nickname, at the time for Johan, the Middle English version of “John.” In modern English it is historically a nickname for John (President John F. Kennedy was “Jack Kennedy” to his familiars) but in recent decades it has been commonly used as a full name by itself.