- 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'.
- There is also a theory that it is Celtic in origin, meaning "Healthy, Strong, Full of Vital Energy", from a putative Ancient British Jakkios.