Green’s Dictionary of Slang

(Irish) the lavatory (cf. jax n.); thus jacks roll, lavatory paper.

1958		(con. 1940s) B. Behan Borstal Boy 154: He wanted to go to the jacks himself.
1961		T. Murphy Whistle in the Dark Act I: And Des went out to the jacks and we heard the scuffle starting.
1975		J. Ryan Remembering How We Stood 107: The man who was to come to fix the jacks, the lavatory.
1981		B. Behan ‘The Catacombs’ in After the Wake 86: I was waiting on the girls to use the Jacks first.
1988		R. Doyle Commitments 1: While Ray was out in the jacks.
1996		(con. 1970) G. Moxley Danti-Dan in McGuinness Dazzling Dark (1996) I vii: Did it make you want to go to the jacks?
1996		P. Howard The Joy (2015) [ebook] In me pocket, there’s a bit of jacks roll, onto which I’ve rubbed some butter.
2005		P. Howard Curious Incident of the Dog in the Nightdress 128: She races over to the jacks, roysh, where there’s an outside hose.
2015		twitter.com 23 Feb. 🌐 Checking out the back of my dress from @LauraJHalton in the fancy jacks at the Oscars .