Jack m
English: originally a pet form of JOHN (SEE John), but now well established as a given name in its own right. It is derived from Middle English Jankin, later altered to Jackin, from Jan (a contracted form of Jehan John) + the hypocoristic suffix -kin. This led to the back-formation Jack, as if the name had contained the Old French diminutive suffix -in. It is sometimes also taken to be an informal pet form of JAMES (SEE James), influenced no doubt by French JACQUES (SEE Jacques). See also JOCK (SEE Jock) and JAKE (SEE Jake).
First names dictionary. 2012.