Don't forget that Jack/Jacky/Jackie are also nicknames for the "Jacob's family", not only for John. In France or in Belgium, Jacky is a very common pet name for Jacques. For example the driver Jacky Ickx was a Jacques and there are many other examples like this. In my opinion, Jack is derived from Jacques and not from John; I don't believe in the Jankin's theory. Jackie used for Jacqueline is one of the examples that gives me reason. Think about it.