Quote:
Derived from Jackin (earlier Jankin), a medieval diminutive of JOHN. It is often regarded as an independent name. During the Middle Ages it was very common, and it became a slang word meaning "man". It was frequently used in fairy tales and nursery rhymes, such as 'Jack and the Beanstalk', 'Little Jack Horner', and 'Jack Sprat'
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Makes more sense as a diminutive if you consider 'Johannes' for John and 'Jankin' for Jack.
http://www.behindthename.com/name/jack