John Roberts, n. — Green’s Dictionary of Slang

John Roberts n.

[John Roberts, MP, the author of the Sunday Closing Act, which was applied to Wales]

enough alcohol to last a drinker from Saturday night to Sunday night.

1886

J.C.Sikes in N&Q Ser. 7 I 306: A new measure of drink, enough to keep a man tipsy from Saturday to Sunday night, is universally known throughout Wales as a John Roberts [F&H].