As a rarer (but older) masculine name it is probably derived from a surname: see Fay 1 or Fay 2.
American writers Jack London (1876-1916) and Jack Kerouac (1922-1969) were two famous bearers of this name. It is also borne by the actor Jack Nicholson (1937-) and the golfer Jack Nicklaus (1940-). Apart from Nicklaus, none of these famous bearers were given the name Jack at birth.
In the United Kingdom this form has been bestowed more frequently than John since the 1990s, being the most popular name for boys from 1996 to 2008.
Famous bearers include the German intellectual Max Weber (1864-1920) and the German physicist Max Planck (1858-1947). This name is also borne by the title character in the Mad Max series of movies, starting 1979.