Green’s Dictionary of Slang

jack n.¹³

the anus.

1945		‘Beaufort’ in Mess Songs & Rhymes of the RAAF 29: With all sorts of horrible pains in our jacks.
1981		S. Berkoff Decadence in Decadence and Other Plays (1985) 30: Shove that up your jack.
1984		Partridge DSUE (8th edn) 607: C.19–20.
1994		B. Fallowell One Hot Summer in St Petersburg 148: He looks alarmed, as though expecting a swagger stick up his jack any second.