Green’s Dictionary of Slang

the lavatory; the anus.

c.1550		T. Ingelend Disobedient Child in Dodsley II (1874) 276: Wilt thou [...] To let thy youth unhonestly be spent, / And do as poor knaves, which jaxes do scour?
1596		‘T.C., Traveler’ (Sir J. Harington) An Anatomy of the Metamorphosed Ajax 11: The great brass sluice [...] sent it down a gallop into the Jax.
1635		A. Read The chirurgicall lectures of tumors and vlcers 187: [N]o Fistula ani is of an easie curation: for this part being the jax of the body, it affordeth great plenty of impurities, and much superfluous moisture.
1684		J. Norris The accomplish’d lady 6: Pamphlets, which Men have scribled against that Sex, do so stink in the Nostrils of Good Men [...] that they are only thought fit to be bound up with the Metamorphosis of A-jax.
1946		‘Myles na gCopaleen’ Drink and Time in Dublin in Irish Writing No. 1 May 74: So I take one [sleeping pill]. But I know the doctor doesn’t know how bad I am [...] So out with me to the jax where I take another one .
1995		A. Higgins Donkey’s Years 30: Coffey crouches like a monkey above the jax.
2002		Blue Pages (Dublin) ‘Dublin Dictionary’ 🌐 Basement Jaxx Downstairs toilet.