'Jack' has been the familiar form of 'John' for at least 800 years. Many people believe it came from the French name Jacques, but The Oxford English Dictionary - the 'bible' in matters of this sort - doubts this. They are much happier with the case - made over a century ago by the scholar EWB Nicholson at Oxford - that the word actually originated in that form as a diminutive for 'John'. In other words, it's just as much a name in its own right as 'John' or any other name is and it did not depend on any foreign influence.