Green’s Dictionary of Slang

jack (in the box) n.

venereal disease.

1954		T.A.G. Hungerford Sowers of Wind 3: Penicillin’ll take care of that. They reckon they just pump you full of it, and bingo! No more jack!
1960		J. Franklyn Dict. of Rhy. Sl.
1974		Adamson & Hanford Zimmer’s Essay 56: No worries about jack in this one. A virgin, no worries.
1979		D.R. Stuart Crank back on Roller 199: Ah, the poor bastard got a jack in Tassie before we’d been in camp a month . . . Fuck a dog on a chain, the bloody fool [AND].
1984		Partridge DSUE (8th edn) 607: since ca. 1870.
1984		B. Ellem Doing Time 191: jack: [...] ‘the jack’ refers to a venereal disease.
1985		N. Medcalf Rifleman 73: Got malaria, beri-beri, malnutrition and probably a dose of jack [AND].
1990		R.G. Barratt ‘Who’s Jack of Robbo?’ in What Do You Reckon (1997) [ebook] I got the jack years ago and had to go to the VD clinic.
1992		R. Puxley Cockney Rabbit.
2003		B. Dark Dirty Cockney Rhy. Sl.