Green’s Dictionary of Slang

potatoes (in the mould) adj.

cold; also as n.

1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier.
1932		‘P.P.’ Rhy. Sl. 7: Blinkin’ ‘pertaters’ this morning George.
1960		J. Franklyn Dict. of Rhy. Sl.
1971		J. Jones Rhy. Cockney Sl.
1992		R. Puxley Cockney Rabbit.
2001		M. Coles Bible in Cockney 21: There is always gonna be potatoes-in-the-mould and heat.