Green’s Dictionary of Slang

apple fritter n.

bitter beer.

1937		A. McLean Vive la Legion 135: ‘Blimey, I’m thinking a run up the apple and pears and a good old apple fritter...’ ‘No good, Marmaduke,’ I laughed, ‘there’s no bitter here,’ for I understood his underworld slang. That little Cockney was just dying for half a pint of good old English beer.
1960		J. Franklyn Dict. of Rhy. Sl. 33/1: apple fritter Bitter [...] It seems to be of post-1914–18 war origin, and is more often used in invitation than in ordering – ‘Come on ole mate – come an’ ave’ an apple-fritter!’.
1971		J. Jones Rhy. Cockney Sl.
1992		R. Puxley Cockney Rabbit.
2000		The Original Pearly Kings and Queens of London 🌐 Apple fritter – bitter.