Green’s Dictionary of Slang

(Irish) generic for Guinness and the family who own it; thus Arthur Guinness talk n., drunken language.

1970		N. Furlong Harp n.p.: Wexford, a city which has close and blissful relations of an impeccable moral standard with our Uncle Arthur to this day [BS].
1995		Irish Times 27 June n.p.: An unfamiliarity with the proper Dublin-brewed pint from Uncle Arthur is probably to blame for some of their condition [BS].
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Arthurs (n): a pint of Guinness. [Ibid.] That’s Arthur Guiness [sic] talking (phr): when someone is talking rubbish while under the influence.