Green’s Dictionary of Slang

jim and jack n.

the back.

1966		Took & Feldman ‘Bona Prods’ Round the Horne 16 Apr. [BBC radio] I see Samson as huge and all butch, with great bulging thews and whopping great lallies, with long blond riah hanging right down his Jim and Jack.
2019		P. Baker Fabulosa 293/2: Jim and Jack a back.