JACKmEnglish 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. American writers Jack London (1876-1916) and Jack Kerouac (1922-1969) were two famous bearers of this name. It is also borne by American actor Jack Nicholson (1937-).