Green’s Dictionary of Slang

(mainly US black) a newcomer or novice, esp. to the fast life of the ghetto streets.

1985		Marley Marl ‘Marley Marl Scratch’ 🎵 He’s not a new jack, he’s been here for years.
1993		Snoop Doggy Dogg ‘Gz and Hustlas’ 🎵 Sit back relax new jacks get smacked.
2000		New Yorker 3 Apr. 🌐 The new guys, rookie officers like me—we were called ‘new jacks’ by the inmates.
2011		G. Hayward Corruption Officer [ebk] cap. 9: All we heard were comments from the Senior Officers. ‘How many of these fuck’n jacks dfid we get!?’.