I mean, you could go with Jackie (as in Robinson)? But remember: Jack itself is a nickname, so it’s hard to shorten.
Famous people named Jack:
Famous Jacks: Jack Be Nimble, Jack in the Beanstalk, Little Jack Horner … oh wait. Novelists Jack Kerouac and Jack London; actors Jack Nicholson, Jack Webb, and Jack Palance; Jack Dawson from Titanic, who damn well should’ve fit on that floating door.
President John F. Kennedy’s nickname was Jack.
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