Green’s Dictionary of Slang

jax n.

also jaxx
[jakes n.1 but later use might be f. jacks n.2 given prob. difference in pron.; later use Irish]

the lavatory.

[UK]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?
‘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.
‘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 .
[UK]A. Higgins Donkey’s Years 30: Coffey crouches like a monkey above the jax.
Blue Pages (Dublin) ‘Dublin Dictionary’ 🌐 Basement Jaxx Downstairs toilet.