Jack

Jack, a familiar form of John, the most widely spread of Christian names, and said to be derived from the French Jacques or, as others maintain, from Jankin, a distinctive form of Johan or John; Johnkin gives us Jock and Jockey; from its extreme commonness it has acquired that slightly contemptuous signification observable in such compounds as “every man Jack,” “Jack-of-all-trades,” “Jack-an-apes,” and the name as applied to the knaves in playing-cards, and to the small white ball used as a mark in the game of bowls is an example of its transferred sense.

Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)